I am not sure if it was the upgrade to High Sierra or the upgrade to the latest version of Omnifocus but the sidebar in my custom theme has changed colours but I haven’t changed anything in the theme.
Not sure what caused the change but before these upgrades the sidebar was the same colour as the content background (#272822).
After more digging, it looks like something is casting a colour on the side bar and sidebar tabs. I can change the colours using the sidebar tabs and sidebar perspective tints like before but unlike before the colours I choose are not what is displayed. The colours are similar though.
Not sure what would be causing this but I didn’t see the behaviour previously.
do you have “reduce transparency” checked in system preferences->accessibility?
i think that could be the issue, though i’m so used to the “reduced transparency” looks that i don’t even remember how it was before…)