Agents app: wrong WCO background color in all themes on windows

[benibenj]

[hawkticehurst]

Reopening since this issue since it more accurately reflects a bug we're still seeing in Insiders (compared to #314395 –– which was about gradient background updates). Also updating milestone, moving verification labels from other issue, and reassigned to @benibenj since he was working on fixing this specific issue to my understanding.