You have to define the thickness of the border, the style (I think only solid is allowed) and the color, which can be in hexadecimal or RGB notation, as well as plain names for simple colors like "blue".
Again, you can define the color in any notation as mentioned above. The "A" in RGBA stands for alpha which is the opacity, expressed as a number between 0 (completely transparent) and 1 (completely opaque).
Just as a reminder, the files inside the extension folder might be overwritten during an update, so you might want to keep a copy or notes somewhere else.
Try to write your own custom styles at the end of the file, as CSS processes rules in order from top to bottom. If there's another rule after yours that applies some styles to the same property, it would take precedence.
Just in case, remember that changes will only take effect the next time that the extension runs. You can either log out and log back in, or toggle the extension on/off from the Extension Manager.
Hm, that didn't work either.
I put my code at the end and I also switched the #panelBox#panel (the border still worked) and #panelBox – I also tried it only with the #panelBox. But no effect.
And yes, I also restarted the whole system.
I uploaded my stylesheet.css – maybe I did something wrong?
Have you set the taskbar to hide (the exact name is Panel Intellihide)? If yes, that might explain it, as CSS rules have this concept of specificity where the more specific you are when applying rules, the higher the precedence.
You can then apply the rules when that is the case. Still at the bottom of the file:
Thank you very much. Yes, I use Intellihide.
It has now almost worked, but now I also have the red background in addition as the background of the panel (quasi behind where nothing should actually be).
I don't think it is a Bug. It is maybe because of the panelBox Value. Because this whole Space could be reserved for it. So, it could color the whole Space. The Thing is: we don't want that space, we want the Taskbar ...
Maybe it would work when You use the Value #panelBox.floating #panel because this for the floating Mode but with Connection to the Panel. Maybe this could be worth a Try.
You might have to check the gnome Shell .css. The .css for the extension should work but... since this is a system of many moving parts - you end up with extension configurations and shell configurations.
The default Zorin Theme likely matches in both locations, whereas your changes are only reflected in one, not necessarily the .css file that is given system priority to.
Summary
This is also why I no longer create Gnome Themes. Between libadwaita, the Shell and extensions; it is just too convoluted now for a user to re-take control of their desktop.