Originally Posted by shimshom
Those colors aren't accessible through theme colors I'm afraid.
The only way to reduce the colorization (you'll still have the border) is by using a special High Contrast theme.
For example, my Astronomer / Night Mode HC theme, uses only red and black and removes all removable colors through theme colors:
What is left, those borders and white text on Desktop cannot be changed.
This is based on the HC Midnight series of themes in the link below.
Browse all the themes posted in this (large) thread, I'm pretty sure Cliff made some interesting High Contrast combinations.
For more info and other themes on high contrast, check out my special themes thread on EightForums.
Win8 themes are compatible with Win10.
Otherwise I'd have a hard time right now transferring and converting all that I once posted.
But for a normal theme without high contrast: you cannot change those ribbon colors without third-party customizations.