In a splash screen, I'd like to change the OK button to a darker grey.

I am only able to do this for the color after clicking the check box:

Here are my theme colors:

That white/light grey one controls just the background. And adjusting any of these theme colors doesn't change the OK button.
Making the background fully white in that theme color, makes the Cancel button in the filter widget worse:

This grey color for the cancel button becomes lighter when the background becomes lighter.
The tertiary theme color seems to control the color of the cancel button but when I make it a darker grey, it just makes the whole button black:

I know these buttons are supposed to be semi-hidden since they are not clickable until the user clicks the checkbox in the splash screen, or after applying a filter to be able to cancel it, but in order to be 508 compliant I am told that even disabled buttons need to follow the 508 compliance rules.
Is there any way to have full customization of these buttons?