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).
Related reports: T92581
#### Before
{F11680317, size=full}
#### After
{F11680225, size=full}