A few months back with Opera 12 alpha launch, Opera added support for themes, yes! You guessed right, they renamed Skins to Themes. You can download Opera themes from here. By installing the theme for Opera, the background image of Speed Dial will be changed and also the colorization. Opera Production team member Michael created Opera Theme Creator add-on, which you can use to dress up upcoming Opera 12 browser according to your liking.
Creating custom theme for Opera
1. All you need to do is download and install OTC add-on for Opera from here.
2. Once installed, visit any wallpaper site offering free wallpapers to download,
3. Click on Opera Theme Creator icon on Opera toolbar, and hover your mouse pointer over an image on the site you’re on, preview will be shown how your Opera theme looks with that wallpaper. You can here choose either best fit or tiled image by clicking on “More options”.
4. Give a name for the theme and also for the Author, once all done, click on “Try out your theme” to apply that theme to Opera.
If I am not wrong, Chrome has an official extension from Google that allows to create custom themes, seems Chrome Theme Creator is much better as it allows to create your own theme from custom image in your Computer, where this Opera extension lacks this feature.
Source: Choose Opera blog
Update: Opera Theme creator extension has been updated and now allows you to upload custom images from your Computer to create themes for Opera.
Update 1: This add-on works in Opera 11 and Opera 12 only.