I\'m looking to create an extension for personal and possibly personal friend usage as well that simply changes a few colors that are displayed on a website I visit. I use Reddi
You can do exactly as you said: you can inject a CSS file or code into the page. The easiest way is through the manifest (look at the CSS property).
You only need to complete your manifest.json
in order for Chrome to know in which pages it has to insert your CSS file into.
If you want to decide programatically when to insert the CSS, you can use chrome.tabs.insertCSS(), where you can insert a CSS file or just the code as a string.