Allow customization of nav menu light/dark theme colors

[bronwyncombs]

From Nate Shoobs @ OSU via discourse:

We’re very happy to have the new features in the 7.12 release. But less enthused about the new color scheme and UI updates. Is there any way to keep the original color scheme?

• return the original dark grey/black sidebar theme as an option in addition to the new “dark”. The brown color used as the “dark” theme introduced in this version is inconsistent with our institution’s web style guidelines.