Me too, I often highlight by left clicking my mouse button, and drag the blue screen over the entire thread. It is irritating, but it helps my eyes.
If you’re on a Windows machine, press CTRL and A together; it automatically highlights all the text. Came in handy when I had my eyes done. To the OP, Dark Reader is the way I go when needed.
On my devices, I can just highlight a word and then click select all when the pop-up pops up. Sometimes I have to hit the 3 dots at the end to get to select all, but it sure beats highlighting everything by hand!
Using
“CTL A” to select a large area may save some time...