I'm still not quite sure what you're looking for. Is this what you want? Could you provide some more detailed example, eg something from Chrome, Firefox or Safari that has what you're looking for?
The post I mentioned includes this as a feature:


You can also set dark/light mode in settings too.
Or, do you mean a high-contrast mode, to increase the saturation of colors to make it better for accessibility?
Is the adjustment of colors for the browser UI, or for websites?