Android is extremely popular largely due to the fact that users can easily modify its look, which applications to use, and basically anything else. Changing your look is not only a wallpaper, launcher, or new icons. Rather, some deep changes can be done with custom themes that are not so hard to create.
With elementary knowledge of photo editing and a willingness to learn, you can create your own theme. XDA Senior Member Ayush Singh wrote a thorough guide on understanding the process of making a theme. The guide initially was aimed at the Sony Xperia Z1, but it can be used with almost every device, as themes don’t differ much in their structure. Ayush Singh presents the process with text and some screenshots, so so you can easily compare them to your own work in progress. Theming brings a lot of fun and can totally change the look of your device. With slight modifications, you can build a theme alongside your personal ROM.
You can find everything you need to create a theme by visiting the guide thread. Make your way there and start reading. And while you’re at it, check out yesterday’s interview with a popular theme creator.