For notes that I have applied a specific cssclass to, I’d like to target and adjust the style in the sidebar listing as well. However, the cssclass does not get applied to the items in the file list in the sidebar. Is there a known workaround/plugin for this?
I don’t think so. If you look at the tree/DOM in the inspector →
cssclasses aren’t applied until you get down to .view-content .markdown-source-view (or .markdown-preview-view). The sidebars are farther up.
I don’t see any community plugins for anything like this with a quick look.
Ok thanks - confirms my findings as well 
Theoretically you could write a rule at the first common element, and use :has(.yourCssClass) on that, and then drill down the sidebar route…
Ooooh that’s a good idea - will give that a shot!