What this does is make every icon the same hue with the sepia filter, then, you rotate the hue towards red. You need to use high saturation to make the colours pop and every icon should be red.
It partially succeeded. All the webs icons with one color as github, virusetotal...are colored in red, but all the webs with more than one color as google, amazon.. are colored partially in red. It as like this code can color only one color of the icon.
There is any way to color all the icons to red or replace them?
if you fully colour images without transparency [i.e. the google/amazon icons] in red, they won't be distinguishable - they'll just be a red circle/square
however, if you still want to do it; put brightness(0) as the first filter to make it black, then an invert(~0.5) to make it not black
also, you can use https://codepen.io/sosuke/pen/Pjoqqp to calculate the exact colour you want (although if it's 0deg red that's not hugely important)
I added brightness(0) and invert(0.5) and as you say they become red circle/square.
I wonder if there is a way to replace those icons as it can done with bookmark icons?
An example of replacement of Amazon bookmark image (gave me by Firefox-gx theme creator Godiesc):
however tab favicons use data uri's (i also don't know if they accept svg - they ought, but i'm not sure)
so you'd need to inspect the tab, copy the uri, and hope it never changes. lemm.ee's, at the moment, is img[src="data:image/png;base64,iVBORw0K[..]8sRwEqwGyXYQAAAABJRU5ErkJggg=="]
using this technique, you should be able to have (obviously use the full data uri for the match, but it's too long for a lemmy comment)
you might want to add extra specifiers so it doesn't match the same image if used elsewhere (i don't know if it ever is)
i.e. img.tab-icon-image[src=".."] {}
however, this doesn't work from my minute or so's testing
(by the way if you add 3 backticks you get a codeblock rather than the ol' ugly inline code. also you can specify the language in markdown, although i don't know if it affects anything in lemmy)
[^1]: also this only works in ffx, but that's not important right now