I maintain an IntelliJ theme (Darcula Sombre), and latest build of IntelliJ 2020.1 introduced a grey background color in editor tab's close button (the gray circle).
Do you know the property I should override? I enabled Internals (add "idea.is.internal=true" to idea.properties) then looked at Tools > Internal Actions > UI > LaF Defaults, but found no useful property. Tested on IntelliJ 2020.1 EAP, Community edition.