Hello,
After switching to Dark Theme on your Site from Theme Settings, you can try using the following Custom CSS to change the Dark Theme color and paste it to Dashboard >> Theme Settings >> Custom CSS >> Global CSS section:
.main-page-wrapper { background-color: red !important; }
In place of red you can write your own color name or hex code.
Also, regarding changing the Pages Background colors, you can go to Dashboard > Theme Settings > Styles and colors > Pages background.
Best Regards