You could choose a theme to use, or you can design you own custom theme. To do this, click on 'More Actions' in the top right corner, and go to 'Manage Site'. Then click on theme at the bottom left. You may choose any of these themes available. If you want to modify the colours in the theme, choose the theme first, and then click on 'Colours and Fonts' to modify it. You need to select the 'custom' button to make modifications.
Some things to try:
It should give a good insight into who you are.