Maybe the theme is setting a transparency on your selection… Otherwise I can’t imagine what could be different. You could use inspect element to check the selected text’s ::selection css
Still can’t repro on dark theme. I’m on Windows Chrome, maybe Chrome is doing something funky on macOS.
Try replacing the color with something like rgba(60, 68, 82, 0.99) and see if that helps. If not I don’t know what else you can try… looks like the browser is doing some weird calculation.
It would be useful to share a screenshot from your devtools too. For example on my end it’s clear that it’s being set by the <style> tag, which is the custom CSS, and it’s overriding the default theme.