You will need to install a user script manager extension to install this script.
Search for text on specific pages and applies a style to the matches.
Multiple rules can be used.
I've being using this script for years to highlight text on certain pages.
IIRC it's based on this script, with small modifications to allow multiple rules.
Rules are added to the script itself, using the following block:
<regex1> matches the URL of the page(s) you want to use this rule
<regex2> matches the desired text. round brackets are mandatory
<style> inline style to use on the matched text
There are a couple sample rules already on the script.
I've seen the script hang a few times on certain pages/rules. It's rare, but be careful.