In order to create themes, you must have the base CSS file that is included with every download of Titan.
The base theme has little comments on what each QT element does, but a fully commented base theme is kept up to date by Bl4ckNeon and can be found here: 
The base theme is a great starting point if you are looking to fully customize and change the Titan layout or if you want to change a single element.
The easiest way to make a theme is to use notepad++ (or your favorite advanced text editor/IDE) and find and replace all of the default colors (stated at the very top of the CSS file). For a more indepth theme, it requires going tag by tag and editing/adding properties.
If you would like your theme to be officially endorsed and even possibly shipped with Titan then the following must be present:
1. The background property for "QPushButton#hamburger_logo:hover" must be #0C9BDC (official Titan blue)
2. The background property for "QPushButton#hamburger_logo_exit:hover" must be #52c175