For the highlighted code, you’ve made unhighlighted lines look more like comments than merely deemphasised lines. One alternative style that I’ve found effective in such situations is to reduce the opacity and partially desaturate:
But I have always found adding a yellow background to the line to be by far the most effective (that is, the most likely to be read and understood correctly without explanation), most likely combined with opacity reduction and/or desaturation. And yes, specifically yellow will normally yield the best results. (There may be cultural factors to weigh against this recommendation, but there’s also some actual colour science supporting it. I’d definitely avoid red and green for normal highlighting in western culture, though it’s great for removed and added lines in diffs—but do remember the colourblind and not make colour the only indication.)
Agreed on highlighting with a different background color. Unfortunately, it can be difficult to find a background color that is different-enough from the usual background while maintaining a high enough contrast with the text sitting on top of it. This is especially true if your syntax highlighting color palette covers all the hues you could use.
So here's another protip: Make the highlight pop with a brighter left-border, so your background color doesn't have to pop so much on its own and you have more wiggle room to maintain contrast. Something like what mdx-prism has in their readme image [1], though I don't like the specific blue-on-blue colors. This blog article [2] has an example in the middle with nicer colors.
