Edit: I see greyelf beat me to it.
Most of Harlowe's styling is on the <tw-story> custom element in Harlowe 2.x, (instead of on the <html> element, as in Harlowe 1.x, or the <body> element, as in most web pages and web apps). Links in Harlowe also use the custom <tw-link> element for most links, and the class .enchantment-link for generated links (i.e. links created via the (click: macro).
So to change the text color across a Harlowe story, you'd use CSS like this:
tw-story {
color: #fdff29;
}
For the links:
tw-link, .enchantment-link {
color: #fa5c5c;
}
To find the selectors for elements, you can look at the html structure and css of web pages and web apps, including Twine stories, by using your browser's development tools. For example, on Google Chrome running in Windows, you can right click on an element and then select inspect element from the context menu, which will open a dock in the right side of the browser window detailing the document's html structure and css code, which can be altered in real time to test things out (these alterations are not saved, however).