You can root your phone with the help of Google. Access Google and then type root model of your phone and brand. Once you find a tutorial, follow it and the phone should be rooted. Notice that you might void your warranty if you plan to do this.
Secondly, you can't apply themes even if the phone is rooted, because themes are part of the operating system. You will have to flash another custom version of the OS in order to have themes. CyanogenMOD has a theme manager that allows you to set custom themes to the look of your phone. I recommend checking XDA Developers in order to obtain additional information.
