As I’ve mentioned in my previous comment - third party themes are not guaranteed to obey the colour scheme, they might but often do not.
If you want to change the colors in a theme that does not obey the color scheme, the theme might have a control panel to let you choose custom colors, but if it doesn’t than the only thing you can do is to edit the theme source code.
One note regarding terms and definitions - Plasma offers different theming options that the can mix and match, but make sure to it use the correct terms so that people understand you:
- “Plasma style” controls the look and feel of Plasma UI elements like the panel and widgets.
- “application style” controls how applications (sth least KDE and Qt apps) look - buttons, tabs and stuff like that.
- “window decoration” controls how the borders around windows, and specifically the titlebar looks.
- “color scheme” controls the colors used in any of these styles - if they support KDE color schemes: all KDE content does but most 3rd party themes do not.
When you talk about the color of the titlebar - you need to talk about the window decoration theme.