LiveJournal offers two style systems to modify the way your journal looks. Style System 1 (S1) is controlled by overrides and the Edit Style page. Style System 2 (S2) is controlled by the Customization area. http://www.livejournal.com/support/faqbrowse.bml?faqid=173 contains information on the differences between the style systems.
If you are using S2, information on customizing your journal can be found at http://www.livejournal.com/support/faqbrowse.bml?faqid=170. S2 does not use overrides. If you are using S2, and you want to customize your journal in a way that your chosen S2 layout does not support, you will need to switch back to S1 in order to use overrides. Information on switching between S1 and S2 can be found at http://www.livejournal.com/support/faqbrowse.bml?faqid=168. Information on S2 customization can be found at http://www.livejournal.com/support/faqbrowse.bml?faqcat=s2.
Basic S1 customization is controlled by the Modify Journal page, http://www.livejournal.com/modify.bml. Here, you can select colors for your journal, either by choosing a pre-set theme or a custom range of colors. Information on which color usage area controls which part of your journal can be found at http://goathack.livejournal.org:8055/docs/usageguide.html, and a basic guide to hexadecimal color codes can be found at http://goathack.livejournal.org:8055/docs/color_codes.html.
The Modify Journal page is also where you can input overrides. Basic information on overrides can be found at http://www.livejournal.com/support/faqbrowse.bml?faqid=124, and more detailed information on overrides can be found in the entire S1 category, located at http://www.livejournal.com/support/faqbrowse.bml?faqcat=s1. Tutorials on overrides and how to use them, along with some commonly-requested overrides, can be found at the HowTo journal, http://www.livejournal.com/userinfo.bml?user=howto.
Paid, Permanent, and Early Adopter accounts can change the entire layout of the page and customize all the HTML by making their own S1 styles. For more information on creating your own style, see http://www.livejournal.com/support/faqbrowse.bml?faqid=128 . If you choose to make a new style, make sure to select the name of the new style from the dropdown list at http://www.livejournal.com/modify.bml in order for it to be displayed on your journal.
If you do not see options on the Modify Journal page other than the ability to select mood icons, you have set your journal to use S2. Please see http://www.livejournal.com/support/faqbrowse.bml?faqid=174 for more information.
You can also set a title and subtitle for your journal, and a title for your friends page, by visiting http://www.livejournal.com/editinfo.bml while logged in. Scroll down to the section labeled "LiveJournal Options" and type in the title, subtitle, and friends page title you would like. (If you would like only one or two of these, simply leave the other box(es) blank.) The title and subtitle will appear in your profile, and the title and friends page title will appear as the title of your journal and friends pages when they are viewed in a browser.