Go to the about:config page in Firefox and try changing the toolkit.legacyUserProfileCustomizations.stylesheets preference to false, if it's not already set to that. The value can be changed by double clicking it or using the toggle button on the right side.

After making the change, restart Firefox. Does the issue go away?

If it does, the issue is caused by a some code that you added to the userChrome.css file in your Firefox profile folder.

That setting was already false I am just normal user i dont know any codes it just happened like this

hello melihblc,

Go to 3-bar menu (or 'Tools') => Add-ons => Themes and choose the Light theme.

I hope its helps.

Thank you