Created attachment 123417 [details] screenshot about the issue SUMMARY STEPS TO REPRODUCE 1. Download arc-theme from repo and set it in KDE Settings -> Application style -> GNOME/GTK Application style 2. Download Robo 3T and/or Postman from snap 3. Open Robo 3T and/or Postman OBSERVED RESULT After opening these programs, you can see they're only using Arc theme's titlebar. EXPECTED RESULT Uniform look for GTK apps, without missing theme elements. SOFTWARE/OS VERSIONS Windows: macOS: Linux/KDE Plasma: Kubuntu 19.10 (available in About System) KDE Plasma Version: 5.16.5 KDE Frameworks Version: 5.62.0 Qt Version: 5.12.4 ADDITIONAL INFORMATION
Forget to mention, but installing these apps from their website solve this issue.
One other thing: just tested this on Ubuntu 19.10/Gnome 3.24, and there's seamless integration between snap apps and the GTK theme.