Domain highlighting

I am currently developing/designing a theme and stumbled across a problem:
normally (for example the dark theme and Alpenglow) the domain gets highlighted by being brighter than the rest of the URL. I couldn’t exactly find out why, but some themes (e.g or one of my themes don’t highlight the domain. Instead the whole URL has the same white color. After playing around with FIrefox Color and the manifest.json for a bit, I found out, that the toolbar color affects the highlighting. If the Alpha value is 0 it gets highlightes. For every other value it seems to be inverse to the Alpha value: 1 means solid color and 100 means normal highlighting. I neither know if this is the expected behaviour nor if this is the place to ask this question. I am also sorry, if this post is a bit vague but I’m trying my best to understand and describe what’s happening. Thank you very much in advance