Installing a theme isn't a smooth process: you download a .crx file, then Google Chrome loads theme and offers the option to go back to the default theme. You can't preview themes before installing theme and there's no interface for managing themes. The only way to uninstall themes, extensions and to reload themes is by typingchrome://extensions/ in the address bar.
For now, the gallery only includes themes created by Google, but you'll be able to create your own theme and upload it to the gallery in the near future. Check how you can create a theme for Google Chrome.
Bonus themes. There are two sample themes that aren't included in the gallery: Camoand Snowflakes, one of the best Chrome themes.
But If you one of those who want a stable browsing experience than don't install Beta or Developer versions and wait for themes to come in Stable version!!!