Remove choice of colors in header

I am running the Grunge Wall theme with the color picker at the top right corner of the header. I’m finding this rather annoying as the site comes up in pink or whatever color it likes, when a browsing customer first opens the blog. ( https://stearn.ca/ )

A lot of browsing readers won’t go through the trouble to check if the color of the site is selectable, if they find it hard on their eyes or offensive they’ll just go elsewhere.

How do I at the very least select a preferred initial color or even remove the color selection ability completely and choose the permanent color manually. Even reducing the choices would be good.

Thx much ?? Will