What are the best tools to inject CSS and JavaScript into web pages?

Absolutely! Tampermonkey is my go-to too. If you want to inject CSS, you can use the `GM_addStyle` function in your scripts. Just set up your user script with the right match URL, and you can manage your styles effectively. Here’s a quick example:

```javascript
// ==UserScript==
// @name Example
// @namespace ex
// @description Example
// @version 1
// @match https://example.com/*
// @grant GM_addStyle
// ==/UserScript==

const styles = `
.foo {
color: red;
}
`;

(() => {
GM_addStyle(styles);
})();
```

You could also go old school with a bookmarklet. Just create one that runs the necessary JavaScript to inject `` and `` tags directly into the page's DOM. Just click it whenever you need those changes!

Tampermonkey is a popular choice for injecting user scripts. It lets you run custom scripts tailored to specific URLs. While it doesn't directly inject CSS, you can easily modify styles using JavaScript to meet your needs.

If you're thinking about using a browser extension, I'd recommend Stylus. It's great for applying custom styles, switching to dark themes, or hiding elements you don’t want to see.