I do not know if you meant this but you do not need the above CSS code in both those files, only in userstyle.css.
The look of Joplin and the editor can be modified using userchrome.css (Red area in the below picture). The look of the viewer pane is modified using userstyle.css (Green area in the below picture).
These two areas use different css element / class names so CSS code written for userstyle.css will not work in userchrome.css and vice versa.
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