Joomla SEO URL’s Fix
This is often a common issue in Joomla and if you google about it you'll find countless questions regarding this and how people have trouble getting them to work. If you've used Joomla you'll know the default URL's it produces are far from SEO friendly with incredibly long extensions, and with the common practice of sharing links on Social Networks it makes it absurd to share a link that stretches for about 100+ characters.
In the backend in the Global Configuration you have the option of setting SEO friendly URL's, use Apache mod_rewrite and adding suffix's to the URL. The latter option is rarely used and it's purpose is to mainly give an extension on that page (so for example instead of "contactus" it'll be contactus.html).
The first two are the options you want to enabled (say yes to), now the most common issue or rather issues is that the CSS gets messed up for any other page than your index if you enabled SEO friendly URL's, and if you also enable the Apche mod_rewrite you get a 404 error. There is a easy fix for this, and while it isn't 100% (some complaints are made regarding it messing up the active li menu item) it's the most effective and the simplest.
First you have to make sure you rename your htaccess.txt to .htaccess, the Joomla htaccess by default is htaccess.txt and you're more than likely to have a default .htaccess on your site. You can either go through your CPanel file manager and change the name or just use your FTP client to rename the file. Make sure you change the extension as if you were to just rename it to .htaccess.txt it still won't work and more so could stuff your website access up.
Once you've renamed the file you need to edit it, either download it your local drive via your FTP client (recommended so if you stuff something up the original file will still be there) or go through your File Manager and edit it, but make sure you make a copy if you're relatively new to this stuff.
There is one little edit to make, if you scroll down your file you'll find a line (see image below)
# ReWriteBase
You simply need to delete the hash (#) in front of it and your SEO friendly URL's will be working. So you'll soon have URL's such as www.domain.com/contactus instead of the old www.domain.com/itemid=$34234tg_?dfdsfdsf based URL's of previous and you will no longer see the 404 error or have issues with CSS not working outside the index page.
This is the quickest fix for this issue and the easiest by far I've come across.
Source (solution given by julian8759)