The Safari updates enabled smart invert. When your screen is in inverted mode the images and video the web pages are "inverted twice" to force them into normal colors. Usually this is desirable but sometimes the images or videos are mostly text, therefore we need them to stay inverted.
Introducing See Better Smart Invert Safari Extension. You can toggle how this this works between smart invert and classic invert. In classic invert, it works like it always did before. You can control it separately for images and videos.
After installing the extension, open Safari Preferences and go to the Extensions at top. See Better Smart Invert will be on the left side, select that and you can change the modes there.
There are some web pages where this does not work yet. One I have found are pages that have twitter images embedded. They are always smart invert regardless the setting in your SEe Better Smart Invert.