Most developers underestimate how difficult it is to allow users to add custom CSS from within the WordPress admin panel. It’s fraught with pitfalls. Making it safe is even more difficult. That’s one reason I’m a fan of the Custom CSS module in Jetpack.
However, adding an extra HTTP request — which, if I recall correctly, effectively loads WordPress twice during a visitor’s first request — is overkill for a few lines of CSS. That request tends to stick out when looking to improve front end performance.
Embed Jetpack Custom CSS is a little plugin to move the CSS into a
<style> tag in the document head to prevent that extra request.