For dark mode i am just using firefox > settings > manage colors + forced same one font for every webpage + disabled usage of webfonts

High LCD backlighting level + dark mode with very dark red background & dark green font and you will be surprised that you can read in both outside and inside environment without changing theme. And you do not lose night vision capability. ( super secret trick from submarine )

I use same colors for KDE theme and for Firefox colors

For lowering brightness of pictures / videos i use extension Dark night mode which is little buggy with paywalls but otherwise works almost everywhere, even youtube.

KDE has Night Light mode but i need to not have red tint colors on pictures.

This is how that looks in practice : https://imgur.com/a/Ob3EhCm