Aha, this isn’t right.
- Global Theme - Breath2 or Breeze is good, Gruvbox and Layan are very nice too.
- Application Style - Breeze is okay, Lightly is better
- Application Style - Configure GNOME/GTK Application Style:
Now here you can set any installed styles. For one misbehaving app, I use a lighter theme (I installed Breeze-Card and Nordic-darker, but Breeze works well if the colour scheme is good with your app - for guayadeque I must use the ‘Breeze-Card’ lighter theme or I can’t see text in a few elements).
To specify a theme NOT part of your ‘global theme’ you can use a specific launcher/shortcut.
For guayadeque I use a mouse gesture to get the light theme applied, launching with the command:
Otherwise, just ‘guayadeque’ would launch it with the desktop settings.
I use ‘Material’ window decorations which follow the colour scheme, I know that Lightly also does this - so that might cure your titlebar.
Of course, you can fix that too…