where to find a color-theme to copy and customize?

root_FileRun/customezables/custom_theme.css

In the settings, select a custom theme

Thanks for fast reply,

I know this file and found it right away, what I really am searching for is a current style theme file, like the red, blue or black theme,  so I can simply copy that css and make changes over there. Normally I inspect site elements in the browser-console but since right-click is not posible i am not able to see what the id's and classes are that beeing used.
 

"css/theme_template.css" is the file you are looking for. Copy that code over to "customizables/theme.css" and edit as you wish. Note that you will need to replace a few placeholders (that look like this: [MAIN], [DARK], etc..) with actual colors.