Themes change the way the site displays on your device.
Responsive themes will change based on the screen size you are using so you can use them on all you devices.
We've made a few responsive themes for your use, one of which is the green theme which is the default.
To change themes, go to Profile -Forum Profile-Look and Layout. The top line shows the theme you are currently using. Next to it is a link that says Change. Click on that and you will get a list of installed themes. The themes have a preview option so you can see how they will look before you change.
Be aware that the Curve theme is not responsive, the smaller the screen, the smaller the text. Enjoy!