XF 2.0 Category background and text color

Specifically the "Block" style properties will help here.

That's Admin CP > Appearance > Style properties > Blocks and probably the "Block header" property.

Worth noting too that the majority of the colours you see in XF are defined in the "Color palette" (under Style properties as well). There's basically 11 base colours which pretty much take care of every single colour used in every single element across the entire style. Changing those is probably a good place to start.
 
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