The color is #f00 which hurts my eyes. How can I change the color of the diagnostic signs and the error message balloon? Didn't find anything yet in the default schema.json

Please checkout :h coc-highlights
I had the same problem, this is what I did:
hi! CocErrorSign guifg=#d1666a
" hi! CocInfoSign guibg=#353b45
" hi! CocWarningSign guifg=#d1cd66
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I had the same problem, this is what I did: