How can one determine the color codes of the existing themes, such that a custom color decoration can be tailored to match?

You had me at 'bring walls back'. As a mild tangent, changing rampart upkeep could interact severely with nukes. At presents people use ramparts (game) as walls (conceptual), as there is little downside. The more exclusive use as a health buffer for critical structures effectively shields elements behind the front line, preventing infrastructure sniping. I think reversing the proliferation of ramparts is a noble goal. A proposition I'd make in tandem to a change to decay rate would be permitting walls incidental to structures, the way ramparts are currently, but not to creeps, which conceptually should certainly be exclusive to ramparts.