CSS, or Cascading Style Sheets, are a way for web pages to associate styling information with their content.
The 'custom styles' option available to Moblog subscribers allows you to add custom CSS rules to pages that display your content - allowing you to totally customise how your moblog looks.
Customising basic elements:
Changing background colour: Add the following to the styles box, adding your choice of colour code: (you can get html colour codes from this page)
body { background-color: #aabbcc; }
Changing header image:
Insert the following in your styles box, changing the height and url values to match your image size and location.
#masthead {
height: 165px;
background: transparent url('http://domain.com/mygraphic.jpg');
}
If you'd like to play with other elements, an easy way to do this is to use the Firefox add-on: Edit-CSS
Please be aware that some CSS may cause your pages to appear 'broken' in some browsers, a good website to advise you on this is www.positioniseverything.net.
You can get more information about CSS in general from W3-Schools