Style nit: Background color and text color of the layer dropdown don't match other header controls

[msnyder-msft]

Describe the bug
The text is not quite as white and the background is a lighter blue color.

Also, the mode tabs don't have borders or rounded corners.

To Reproduce
Steps to reproduce the behavior:

1. Go to Viewer
2. Look at the layer dropdown

Expected behavior
All the header control styles should match

Screenshots

[BrennaCourtney]

The layer dropdown is only grayed out for me in the case that nothing else can be selected (as in Default layer here) or when no layers are selected -- seeming to indicate no actions to be taken/have been taken -- else the color is the same.

Build/View also seem to match the home button/scene change dropdown:

While it's not the prettiest with a dark background it does seem consistent with "these buttons take you out of the current scene" -- @azimamansuri not sure tho