I'm not surprised.
You have many modified "***.css" templates. Try not to make changes in them, and move all the changes in "EXTRA.css".
Disadvantage - some CSS-classes in the styles will be twice - the original and your modified.
Often need to specify the "!important" for some properties. (I still think it's lack of XF, but the developers are looking at it differently.)
The great advantage of - much less modified templates and fewer problems during the upgrade.
For example you can see template "EXTRA.css" in my style
Xenia.