>>75036614chan has a custom css section under Settings > Miscellaneous > Custom CSS [Edit]
For any site in general I use the Dark Reader extension in Chrome, which has a Dev Tools section. You can modify CSS there by domain. e.g. reddit
================================
reddit.comINVERT
header a[href="/"] svg:nth-child(2)
CSS
._1vyLCp-v-tE5QvZovwrASa {
background-color: #000000 !important;
}
._1ump7uMrSA43cqok14tPrG::before {
background: transparent !important;
}
._1OVBBWLtHoSPfGCRaPzpTf, _3nSp9cdBpqL13CqjdMr2L_ {
width: 1000px !important;
}
._1b1Jalg2nxA_Z-BjKXRfAV {
display: none !important;
}
._1vYrJH5uc57mZQJPN4l34E {
display: none !important;
}
._3RPJ8hHnfFohktLZca18J6 {
display: none !important;
}
.avKlJzxZU3brq4nAX0_i1, _1G4yU68P50vRZ4USXfaceV, _2QeqBqfT5UbHBoViZUt-wX {
display: none !important;
}
.PSG6gLueWddB1kqvaOxoT {
display: none !important;
}
._1poyrkZ7g36PawDueRza-J, _2uazWzYzM0Qndpz5tFu3EX {
background-color: #111111 !important;
}
._23h0-EcaBUorIHC-JZyh6J {
background-color: #1a2624 !important;
}
================================
