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 https://addons.mozilla.org/de/firefox/addon/animated-traininrainby1041uuu/ or one of my themes https://addons.mozilla.org/de/firefox/addon/redwavesdark/) 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