The background naturally has a big impact on your website given its size and the fact that it is prominent on nearly every page of your website. As such you need to make sure it is perfectly suited to the role and creates the correct aesthetic for the site.
First of all, you will need to navigate to the system category on magento, and then select the configuration option.
From there you will see a screen similar to the one below. You'll want to go down the lefthand column and select the "framework configuration" under the header of "HelloThemes".
From here you will see the same screen as below. I have annotated it with coloured boxes to show the areas that are key to changing your background. These are as follows:
Blue: Global background colour, this will change the colour of the background on all pages. This is best used with a colour that you want to prominently feature on your site.
Purple: Individual background headers, this allows you to change the background/colour/theme for individual parts of the site i.e the footer or font colour.
Green: Example of an individual background setting, this will apply to only the top links of the header and not the entire header, unlike a global option.
Orange: Save config, this is naturally something that needs to be done as otherwise the config changes will not be saved and time will be lost.