SUMMARY As in subject. After apply "Breeze Dark" theme in Firefox, its start page doesn't become dark. Whilst in case of "Dark" theme (probably built in Firefox) it happens. STEPS TO REPRODUCE 1. Install "Breeze Dark" theme in Firefox 2. Apply "Breeze Dark" theme in Firefox 3. Open "Start page" (as default) OBSERVED RESULT "Start page" has white background. EXPECTED RESULT "Start page" should have dark background. SOFTWARE/OS VERSIONS Linux/KDE Plasma: YES (available in About System) KDE Plasma Version: 5.20.5 KDE Frameworks Version: 5.78.0 Qt Version: 5.15.2 ADDITIONAL INFORMATION Tested with Firefox 85.0
Notice please that in case of Manjaro's themes it works well. I tested with "Manjaro Breath Dark" and "Beautiful Manjaro Dark", "Material Manjaro Dark".
Firefox's 3rd-party "Breeze Dark" theme isn't a KDE thing and isn't connected to the systemwide Breeze Dark theme shipped by KDE. You would have to report this to the creator of that Firefox theme.