Thanks to the plugin Xi/Tweaks it is possible to change the CSS of the tab-title of a tab that is in focus. It works for all panels, so editor, sidebar and message window. For more information visit https://github.com/xiota/geany-preview/issues/5#issuecomment-2016831500.
To change only in the sidebar the CSS of the tab-title of a tab that has focus, visit https://github.com/xiota/geany-preview/issues/5#issuecomment-2016850315.