Apple is introducing a new Liquid Glass look for macOS Tahoe, featuring transparent elements that allow certain sections of the screen to show layers beneath them. If you’re not fond of this change or find it affects your screen visibility, don’t worry – there’s an option to reduce transparency.
In System Settings, go to Accessibility and select the Vision section, then Display. Look for the Reduce Transparency setting and flip the switch to activate it. Note that this setting was previously available but is now more useful due to the Liquid Glass effect.
The reduced transparency feature can be seen in the menu bar before and after applying the change. This new setting gives you more control over your user experience, and Apple has made adjustments to ensure settings are not dimmed as they were in previous versions.
Source: https://www.macworld.com/article/2858361/how-to-reduce-the-liquid-glass-transparency-effect-in-macos-tahoe.html