Problem styling Thundebrid with CSS

Hi, I would like to style the Calendar inside Thunderbird (115.11.1 on MacOS) without using extensions/add-ons. Specifically, I would like to modify the looks of recurring events. I enabled userChrome.css (toolkit.legacyUserProfileCustomizations.stylesheets) and the “has:” selector (layout.css.has-selector.enabled). In my userChrome.css, I added styles:

li:has([alt*="Recurring"]) { background-color:grey !important; padding:1rem !important; }

li:has(*[alt*="Recurring"]) { background-color:yellow !important; padding:1rem !important; }

Unfortunately, this seems to have no effect upon restarting Thunderbird. However, when I open Developer Toolbox (Tools > Developer Toolbox > Developer Toolbox > Inspector tab) suddenly everything works and the stylesheets are applied, but only until Thunderbird is shut down.

Is this intended behavior or a bug?

@adam.jan.piotrowski, discourse doesn’t have an active Thunderbird community. But there is significant css discussion at Suggest you post there.