style: Simplify default background on upgrade banner
Use a solid color instead of a gradient, make sure it works with dark variant.
Split from !1088 (comment 1306691)
Before:
After:
The mockup used a gradient, but it also used GTK3 style.
Use a solid color instead of a gradient, make sure it works with dark variant.
Split from !1088 (comment 1306691)
Before:
After:
The mockup used a gradient, but it also used GTK3 style.