I am working on a gtk theme but didn't get a way to change the color of the panel.
Bonus point for reven.
How to change panel color and other panel style propeties?
Open the default theme's files and look through there, use a color picker to identify the hex color and Ctrl + F 
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I'd rather people look at Plata as a more modern example, the internal theme isn't developed, it's more likely to be using more recent classes I've added in Budgie 10.5 series.
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Plata has budgie-panel class in code, but I can't find that class in the compiled CSS in /usr/share/themes/Plata. Also adding budgie-panel class not working for me.