I'm using Windows 10 In 'Options' (which is the same as 'Preferences' in a mac) The left pane selection text/icon eg: General - if selected - will use blue colour to indicate what is selected.

All checkmarks in selected checkboxes are also blue.

This is the same regardless of using Light or Dark theme.

Do you see the same problem if you restart Thunderbird in Safe Mode ?

What OS are you using?

Using linux. I found a crude workaround - I temporarily switch foreground and background colors. Obviously this is a PITA and not a long term solution as this will be necessary whenever a preferences change is needed. I spent a lot of time researching how to do this via userChrome, but was unable to find anything. Dark Theme isn't anything unusual or exotic so it should really be a built-in option which is properly supported. TB waving off responsibility by blaming the theme is a bunch of crap. End users shouldn't be forced to spend so much time dicking around with something that should be a fundamental feature that has undergone a basic level of QA. Sorry for venting, but this sort of thing really gets under my skin. I guess it's a case of the old adage "you get what you pay for".