Is it possible to override a Brush Key for one theme only?

Hello,
We are overriding some brush keys in our app to improve readability of checked buttons. For example the ToolBarButtonBackgroundCheckedNormalBrushKey is a very light gray and does not stand out well against its background typically, we override this brush to a light blue color so that there is more contrast against non-checked buttons.

This works great for Light theme but in a Dark theme our override is too bright and our white icons are difficult to see. Is there a simple/straightforward way to specify an override per theme (e.g. not override this color for Dark theme, only Light)? Or do we need to go down the route of generating completely new custom themes to have this capability? Any advice would be great.

thank you
Alex George