so first: please, when facing a bug, update first to latest version. It may have been already fixed.
Then: black_pearl.css is a bootstrap variation. So you can change it via the option "Theme Variation".
Remember you can set the option at 3 levels: Global (from theme list), Survey Group, and Survey level.
If you changed an option at survey level, then the changes made at global level have no more effect to this options. You must set it to "inherit" again if you want the setting of the parent to be applied.
I'd suggest you read this article:
manual.limesurvey.org/New_Template_System_in_LS3.x