If you want 'Customize', right-click on a toolbar and select "Customize Toolbar". If you want Settings, click 'Settings' on the 3 bar menu button. If you want themes, select "Add-ons and Themes" (shortcut ctl+shft-a) from the menu.

If you have trouble resolving the problem, this may help: https://support.mozilla.org/en-US/questions/1364418#answer-1474290

You can change this pref on the about:config page from 3 to 2 to revert to the Firefox 94 behavior.

• layout.css.prefers-color-scheme.content-override [dark:0, light:1, system:2, browser:3]

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox