UI: Use theme colors setting for Projectors too

Don't override the theme color settings for the Projector.
If required, the background color of the projector can be changed via

OBSProjector {qproperty-...}

record placed after "OBSQTDisplay {qproperty-...}" section in the theme
file (qss).
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f