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WPF Ribbon Headers not showing

I meet a problem with Fluent Ribbon. I didn't change anything in XAML, but all Headers of my buttons disapeared. I don't know if it is a bug, or simply a shortcut to change appearance and Hide/Show headers.
Here is how looks the ribbon :
And how it looks like from now :
I tried to restart solution but nothing changed, same if I restart VisualStudio. But if I load a backup of solution last week all is working fine, that's why I think there is maybe a shortcut to make them appear/disappear?
Edit :
After additional tests I can add :
Issue is present in both Debug and Release mode
Problem is on all the solution, in all windows using Ribbon, not only that one

If the code is OK, close the project and clear the cache. After that do not forget to delete the folders "bin" and "debug" and "release". Open the project, do a rebuild. Hope to help.

I finally decided to restart from a backup, and modify again the code file after file to finally find the issue (apologies to whom I said that I didn't modify xaml, didn't remember about that one, and didn't notice inmediately that this caused a problem).
In my App.xaml file I use the following style :
<Style TargetType="ToolTip">
<Style.Triggers>
<Trigger Property="Content"
Value="{x:Static sys:String.Empty}">
<Setter Property="Visibility"
Value="Collapsed" />
</Trigger>
<Trigger Property="Content"
Value="{x:Null}">
<Setter Property="Visibility"
Value="Collapsed" />
</Trigger>
</Style.Triggers>
</Style>
<Style TargetType="TextBlock">
<Style.Triggers>
<Trigger Property="Content"
Value="{x:Static sys:String.Empty}">
<Setter Property="Visibility"
Value="Collapsed" />
</Trigger>
<Trigger Property="Content"
Value="{x:Null}">
<Setter Property="Visibility"
Value="Collapsed" />
</Trigger>
</Style.Triggers>
I had to remove all the block and all is working again.
But then I don't understand why Ribbon headers are influented by the style? Well I guess the headers are in textboxes, but they are not empty?

Give a toggle button the color "#FF5D0000" in the main window

I want to give mij text on my button a customized color in the MainWindow.xaml.cs
Normally you give the color in the cs file by this way to the command:
ToggleButton.Foreground = Brushes.Green;
But I want to give the hexnumber
I've already tried something like this :
SolidColorBrush Owncolor = (SolidColorBrush)(new BrushConverter().ConvertFrom("#FF5D0000"));
ToggleButton.Foreground = Brushes.Owncolor;

Instead of doing it in code behind (unless you have a very specific reason to do that), you can work on your xaml
<ToggleButton Foreground = "#FF5D0000"/>
if you are doing it based on some condition, also please take a look at this. It's always a better practice to handle graphical stuff in your xaml as much as you can
for instance you can do
<ToggleButton>
<ToggleButton.Style>
<Style TargetType="{x:Type ToggleButton}">
<Style.Triggers>
<Trigger Property="IsChecked" Value="True">
<Setter Property="Foreground" Value="Green"/>
</Trigger>
<Trigger Property="IsChecked" Value="False">
<Setter Property="Foreground" Value="Red"/>
</Trigger>
</Style.Triggers>
</Style>
</ToggleButton.Style>
</ToggleButton>

Usually in WPF you tend to use styles defined in XAML to change how controls looks. However, using the BrushConverter works if you absolutely have to use the hexadecimal syntax in codebehind. I'd consider building a new SolidColorBrush with Color.FromArgb easier, but that also works.
As for how to use styles and XAML properly, you should probably read some tutorials or books. WPF is quite a different beast than Windows Forms or a lot of older UI frameworks, so there's some re-learning required.
The simplest way of achieving what you want (a different text colour when the button is pressed) would be the following style:
<Style TargetType="ToggleButton">
<Setter Property="Foreground" Value="#FF5D0000"/>
<Style.Triggers>
<Trigger Property="IsPressed" Value="True">
<Setter Property="Foreground" Value="Green"/>
</Trigger>
</Style.Triggers>
</Style>
When placed in the Resources of your Window it would apply to all ToggleButtons in that window.

Dealing with WPF Menu HeaderStringFormat and Access Keys among other things

Okay. So I want my application to display in its main menu the "Save" and "Save As..." items just like Visual Studio does; i.e. "Save {current file}" and "Save {current file} As..."
I would also like to have the normal access keys ("S" and "A", respectively).
I've come up with two solutions, but neither is very desirable.
Instead of creating the main menu exclusively in xaml, I could
create it all in the MainWindowViewModel so I'd have full control over what goes into the generated MenuItems. However, I feel that this would be a violation of MVVM (which I'm attempting to abide by very strictly this time around) as I would have to include references to each MenuItem's Icon in the ViewModel. Plus it seems a little messy.
I can stipulate the header of just these two specific MenuItems (and perhaps future ones) like so, but then I end up getting a MenuItem that not only has a underscore in the header, but also does not contain an access key.
<MenuItem Header="{Binding CurrentFileName}"
HeaderStringFormat="Save {0} _As...">
What should I do?

Whelp, figured it out. At least about how to get it done with the whole main menu described in XAML. Just made the header content an AccessText control instead of a string and it works like a charm.
<MenuItem>
<MenuItem.Style>
<Style TargetType="{x:Type MenuItem}">
<Style.Triggers>
<DataTrigger Binding="{Binding HasSelection}" Value="false">
<Setter Property="IsEnabled" Value="false"/>
<Setter Property="Header">
<Setter.Value>
<AccessText Text="Save Selected File _As..."/>
</Setter.Value>
</Setter>
</DataTrigger>
<DataTrigger Binding="{Binding HasSelection}" Value="true">
<Setter Property="IsEnabled" Value="true"/>
<Setter Property="Header">
<Setter.Value>
<AccessText Text="{Binding SelectedFile.Filename, StringFormat=Save {0} _As...}"/>
</Setter.Value>
</Setter>
</DataTrigger>
</Style.Triggers>
</Style>
</MenuItem.Style>
</MenuItem>

WPF data validation is overriding theme on the interface

I built a WPF application and manage to get the validation working thanks to posts on stackoverflow.The only probblem i'm having is that it's overriding the theme i'm using.
example the theme makes the textboxes look like a round rectangle but after setting the binding it look like the default textboxes. here is my code :
<Button.Style>
<Style TargetType="{x:Type Button}">
<Setter Property="IsEnabled" Value="false" />
<Style.Triggers>
<!-- Require the controls to be valid in order to press OK -->
<MultiDataTrigger>
<MultiDataTrigger.Conditions>
<Condition Binding="{Binding ElementName=txtEmail, Path=(Validation.HasError)}" Value="false" />
</MultiDataTrigger.Conditions>
<Setter Property="IsEnabled" Value="true" />
</MultiDataTrigger>
</Style.Triggers>
</Style>
</Button.Style>
code behind is:
//Form loaded event code
txtEmail.GetBindingExpression(TextBox.TextProperty).UpdateSource();
I've tried to look into the theme file but i was quickly lost.i thought i could use that file like a web css file.Now i've disabled the data binding because of that.Is there any work around for this? thanks for reading this

Not sure if that's the root problem, but try adding BasedOn="{StaticResource {x:Type Button}}" to style element.
Button.Style>
<Style TargetType="{x:Type Button}" BasedOn="{StaticResource {x:Type Button}}">
...

How do I know whether to use a style or override a control template?

This question is inspired by this recent question and other situations I've encountered in my WPF development. How do I know whether it is enough to set a style on a control to override some default behavior vs creating a new control template?
More concretely, in the question above, the author wants to change the look of a ListBoxItem when it is selected. (See code reprinted below). Everything works, except the Background property. How is one supposed to know that they should override the Control Template for this?
<Style TargetType="{x:Type ListBoxItem}">
<Setter Property="Content" Value="{Binding Path=Name}"/>
<Setter Property="Margin" Value="2"/>
<Style.Triggers>
<Trigger Property="IsSelected" Value="True">
<Setter Property="FontWeight" Value="Bold"/>
<Setter Property="FontSize" Value="18"/>
<Setter Property="Background" Value="Yellow"/>
<Setter Property="Foreground" Value="Red"/>
</Trigger>
</Style.Triggers>
</Style>

As to whether to use a style or template Ray provided a great response.
As to how to solve your problem without creating a template, maybe I can help.
The background color is being set by the SystemColors. Using Blend and creating a template you can see the exact xaml.
So if NO TEMPLATES! is a requirement you can always change what that resource is.
Example :
<ListBox>
<ListBox.Resources>
<SolidColorBrush x:Key="{x:Static SystemColors.HighlightBrushKey}"
Color="Yellow" />
<Style TargetType="{x:Type ListBoxItem}">
<Setter Property="Content" Value="{Binding Path=Name}"/>
<Setter Property="Margin" Value="2"/>
<Style.Triggers>
<Trigger Property="IsSelected" Value="True">
<Setter Property="FontWeight" Value="Bold"/>
<Setter Property="FontSize" Value="18"/>
<Setter Property="Foreground" Value="Red"/>
</Trigger>
</Style.Triggers>
</Style>
</ListBox.Resources>
<ListBoxItem>Test 1</ListBoxItem>
<ListBoxItem>Test 2</ListBoxItem>
<ListBoxItem>Test 3</ListBoxItem>
</ListBox>
That will give you the background color for that given ListBox and not screw up anything else in the app.

Styles can be thought of very closely to CSS styles in HTML. If all you want to do is change the basic properties of a control such as Background, Foreground or whatever properties it exposes then a Style is exactly what you need. Styles also allow you to apply triggers so for animations, a style is also sufficient.
If you're finding you want to change the intrinsice behaviours / inner workings on a control then a control template is what you want. For example, if you want to change how a button is laid out by adding some sort of grid behaviour, then using a control template is the way forward.

Unfortunately, for your specific example, you don't know unless you try it. Basically you first try it with a Style....and if that doesn't work for whatever reason, then you write a ControlTemplate. You usually only end up writing ControlTemplates for the reasons Ray mentioned.
My guess is that the trigger you're trying to set has also been hardcoded in the ControlTemplate...which is bad design imo because it prevents the Style from overriding it.

By "Background" I take it to mean the "blue" rectangle that surrounds the ListBoxItem when it is selected?
This is actually the FocusVisualStyle property, which is a style that describes what the item should look like when it is focused. The Control explicitly sets this property (described here), so in order to override it, you will have to redefine the Control Template, making sure to use a default Style setter to set it to {x:Null}.