You should be able to do some adjustments with Themes, and Palettes.
yoursite.tld/admin/customize/themes/
and /admin/customize/colors/
example: I’m using Sam’s Simple Theme, with slightly customized palettes
…and now that I’m trying to screenshot our colours…I can’t find it. Unread links are #7E97CD
but that colour isn’t used in my palette - it’s baked into the theme.
So hoping to be helpful I’ve now got the same/similar question…does it involve editing css or cloning/importing my own version of the theme?
Seems one needs to edit color_definitions by cloning the theme (exported theme in my case) since the setting isn’t exposed in the UI.