The VSE grid theme setting is currently used for two things:
* Indicate time intervals
* As separator between channels
This adds visual noise because for the time interval to be visible, the
grid color needs to be bright, resulting in the rectangle-grid backdrop.
Recently the VSE got have a theme setting to customize alternate-row background
color. This should be sufficient to tell the channels apart without the need
for a line in between.
Additionally, this patch makes the VSE background use the theme setting
as-is, without hard-coded darkening, to ease the tweaking of themes.
This aligns the style of the VSE backdrop with the rest of Blender (Outliner
rows, File Browser, Spreadsheet, Info and animation editors).
#### Before
{F11680224, size=full}
#### After
{F11680225, size=full}