The basic idea:
Members of planet minecraft can change their profile theme color instead of being white or black depending on what color setting the view is on, a user can select in their profile banner editing a theme color.

What does this do anyway?
So basically when a person clicks on the dropbox for 'PROFILE COLOR' they can select a color or put in a hex code for a custom color, when this happens the widgets and theme turns to that color the color will be brighter or darker due to which color setting they are on (obviously the lighter-darker thing would either not be added due to how much code would have to be put in or would only work for the pre-set colors).

Are there complementary colors?
I would say possibly, If your looking for complementary INSIDE of the widget probably not because text color can be edited already as can the font using CoolTextFonts , But outlines for the widget possibly it all comes down to the coding and how much work would have to be put into it, but I'd say if it would there would be another drop-box for complementary colors.

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These are all the pre-set questions I had in mind when I made this if you have more just comment them below, make sure to upvote it and follow it for its updates :D