Theming readme update

GitUI/Themes/README.md

@@ -6,58 +6,66 @@ a default (operating system defined), a bright or a dark themes.
 There is a [Wiki page on Dark Theme](https://github.com/gitextensions/gitextensions/wiki/Dark-Theme)
 with some background information and a list of known issues.
 
-# Themes location
+## Themes location
 
 Themes are stored in *.css files. Where Git Extensions looks for theme files depends on whether it
 is an 'Installed' or a 'Portable' version. Both come with a predefined set of themes in
-`(App install)\Themes` directory.
+`{App install}\Themes` directory.
 
 In addition the Installed version looks for user-defined themes in
 `%AppData%\Roaming\GitExtensions\GitExtensions\Themes` directory.
 
 For each found theme file `ColorSettings` page will show an entry in the Theme dropdown list.
 
-## Installed GitExtension application
+### Installed GitExtension application
 
-### Official themes
+#### Official themes
 
-The Installed version of Git Extensions stores themes in `(App install)\Themes` directory.
+The Installed version of Git Extensions stores themes in `{App install}\Themes` directory.
+The default theme is in `Themes\invariant.css`.
 
-### Custom themes
+#### Custom themes
 
 Any modifications for personal use should be saved to
 `%AppData%\Roaming\GitExtensions\GitExtensions\Themes` directory, because any changes to
-`(App install)\Themes` directory will be lost as soon as user updates or re-installs the app.
+`{App install}\Themes` directory will be lost as soon as user updates or re-installs the app.
 
-## Portable application
+### Portable application
 
 The Portable version of Git Extensions does not have a separate directory for user-defined themes.
-It only uses pre-installed themes from `(App install)\Themes` directory.
+It only uses pre-installed themes from `{App install}\Themes` directory.
 
-# How to create a custom theme
+## How to create a custom theme
 
 Copy `Themes\dark.css` or `Theme\bright.css` and change color values. Use a text editor, preferably
 with .css syntax highlighting and inline color display.
 
+Currently only application colors like branch labels in the revision grid and
+diff colors can be changed, the system colors listed in the templates will not be changed.
+Furthermore a dark theme is only usable if the Windows system theme is dark.
+(Dark mode in Windows settings is not sufficient to change system colors to be dark.)
+
 There is a plugin for Visual Studio to show .css colors inline.
 
-JetBrains Rider displays .css colors inline out-of-the-box.
+Visual Studio Code and JetBrains Rider displays .css colors inline out-of-the-box.
 
-## Create a slightly modified theme
+### Create a slightly modified theme
 
-Use .css import directive to reuse color values from another theme
+Use .css import directive to reuse color values from another theme.
 
 - To import from a preinstalled theme:
+
 ```css
 @import url("dark.css");
 ```
 
 - To import from a user-defined theme:
+
 ```css
 @import url("{UserAppData}/dark.css");
 ```
 
-## Specify alternative color values for colorblind users
+### Specify alternative color values for colorblind users
 
 To keep the number of themes small, color variations for colorblind users are specified without
 creating a separate theme. See for example `bright.css`: