Here you can choose the colours of your website. Aswell as choosing a background colour you can also choose a foreground colour which defines the 'front' colour in the website.  This could be the border colour, surrounding box colour or not used at all - it all depends on which layout you're on. 

Step 1:
Click on the 'design' tab, select the 'customise theme' option and then select the 'website colours' button.

Step 2:
To choose a Background colour for your website press the Choose Colour button beside the Background Colour Field. This will bring up a colour picker where you can choose the colour you want. Once you are happy with it press the select button which will enter the chosen colour into the field.

Step 3:
Repeat the process for the foreground colour.

Step 4:
Press the Save button at the bottom of the page to save your changes.