JITMs style - harmonize with admin theme

[PanosSynetos]

JITMs in Woo Express have

• a gradient background
• black CTA
• green border (left)

We should harmonize them and follow the admin theme colors.

cc @manospsyx for prioritization

[manospsyx]

A quick mockup of how this could look in Woo Express:

Ideally, this should be fixed in Jetpack though.

[PanosSynetos]

What do you think harmonizing them as the following one, and follow the admin colors?

Bonus points: Set a fallback color in all styles, wherever a variable is used
var( --wp-admin-theme-color )