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The following is a dreaded CSS rule from a customized system-wide theme (in Linux) - gtk.css (gtk-3.20). I tested - adding it to A COPY of geany.css; and placed at the end of the file that's in the (Linux) path: home/<user-name</.config/geany/geany.css.
I keep repeating `Menu->Tools->Configuration Files->geany.css`. That handles all the platform dependent paths for you, if a user config version of the file exists it opens that, if not it creates a copy of the system file, commented out so it won't override it, and sets the save path to the user config directory so it won't be overwritten when Geany is upgraded.
If you want to change something that is set in the system file uncomment it and make the change, otherwise just add whatever.
Changes in geany.css in the path shown (in Linux) will override the same code found in geany.css from the INSTALLATION folder (if the code exists). If that code DOESN'T exist in orig. geany.css, assuming the CSS selector plus the property & value that we added are correct, it will still style that element.
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In general, it changes the top border(s) of selected tabs in most (not all) apps that use CSS to style that object in the UI. Not ALL of THE UI in ALL APPS, is styled 100% w/ CSS. It "depends."
In Geany AFAIK the only UI element that isn't standard GTK is the edit widget, so everything else should be CSSable. Sadly GTK CSS negligently omitted classes for every lexical element of every filetype so the edit window cannot be styled with CSS ;-P
If one wants to change an object *ONLY in Geany* or in a given app, they may have to edit an existing <app-name>.CSS file - or create it. It's a BAD idea to edit the original CSS or ANY ORIGINAL file in an app.
User config geany.css overrides Geany system css file(s) overrides GTK user or system settings and themes, but _only_ for Geany. There was some "discussion" about adding the geany.css to the geany config since everything could technically be done with standard GTK methods. But having it in Geany system data dir and Geany user config dir makes it a location Geany can find with `Menu->Tools->Configuration Files` on any platform with any installation of GTK, and it is only loaded by Geany so it cannot accidently affect any other app, and the user needs no platform specific location knowledge.
In Linux, I have: /home/<user-name>/.config/geany/geany.css file. Probably copied from the Geany installation folder to my user folder. (Same concept in Windows, but the path where the geany.css file (to be customized) should go will differ. Where to place it - should easily be searchable (or stated in the Geany user manual).
https://www.geany.org/manual/current/index.html#configuration-file-paths and https://www.geany.org/manual/current/index.html#tools-menu-items
Perhaps it could be improved, contributions are welcome.
That way, if you f.., uh, mess up the edited file, the ORIGINAL file in the installation folder is untouched.
And as I said above, if you edit a Geany system file it will be overwritten next upgrade, just don't do it.
Cheers Lex