Replace raster image gradients (before base64 embedded) for modern
browsers by CSS gradients. Only relying on conventional image URLs
in browsers that don't support linear-gradient.
Also
- adding a darkened border for inactive tabs to harmonize visual
perception of border as one continuous line and
- DRYing tab separator selectors, saving ~8 bytes gzipped.
Bug: T63099
Bug: T121730
Change-Id: I76d32b84ddff06a2c7ef983e6f89ca6e74257a67
Volker E