Go to menu ≡ -> Add-ons and Themes -> Themes. Check your theme.

What theme are you normally using? Is that a built-in theme or a theme downloaded from the Add-ons website or do you use Firefox Color ?

Disable your extensions, start with anything related to themes. Dark Reader has a setting that alters the browser theme for example (Dark Reader > More > Change the browser theme > Default).

Thank you for giving me a hand.

Tried Lush-Soft (somewhat mint color) and Google Chrome (bluish color) themes, which are Add-ons. However, even if Firefox Color theme is chosen, the issue persists.

And the Dark theme is supposed to be disabled but in fact it doesn't work after restarting Firefox. Additionally, I have to press "Disable" and then "Enable" button to reinstate Lush-Soft theme (for example).

Try disabling all your extensions.