Updated the theming guide following matomo-org/matomo#7951

docs/theming.md

@@ -45,9 +45,12 @@ To change the font you override the font variable:
 @theme-fontFamily-base: Verdana, sans-serif;
 ```
 
-You get the point. It is a common convention among themes to put all color values in a `_colors.less` file. This is already done for you if you have created your theme using the above mentioned console command. Just uncomment the variables you want to change. For a list of available variables have a look at the [Example Theme plugin](https://github.com/piwik/piwik/tree/master/plugins/ExampleTheme/stylesheets).
+You get the point. The list of all variables that you can override is defined in the Morpheus plugin:
 
-Please note in case you are not using the theme generator: overriding variables will only work if they are defined in a less file that is imported by another less file.
+- [base theme variables](https://github.com/piwik/piwik/blob/master/plugins/Morpheus/stylesheets/theme.less)
+- [advanced theme variables](https://github.com/piwik/piwik/blob/master/plugins/Morpheus/stylesheets/theme-advanced.less)
+
+Please note in case you are writing Less rules outside of `theme.less`: overriding variables will only work if they are defined in a less file that is imported by the main `theme.less`.
 
 Although we do not recommend to do so, you can change the colors of a specific element directly, for example: