This patch includes the following tweaks to the default theme.
To make reviewing easier, this is just one patch, can be committed
by parts to make it easier to revert later on just in case.
- FIX: Graph Editor header to match Dopesheet's.
- NLA Sound and Meta strips to match VSE's.
- Lighter marker lines, to tell them apart from grid lines.
- Subtle Gradient Vignette in 3D Viewport, helps to lead the eye to the center and read corner labels better.
- Data type correspond with modes. (See T74358)