It depends on your theme. With most themes you’ll have to manually edit the CSS to do this.
But some themes have control panels that allow you to make changes to the design without editing the code. If you’d rather not write code but would like to make changes to your site, I’d suggest using a theme with design options.
Some are very simple, just a few color choices or layouts to choose from, but others have hundreds of options. There are some free ones in the theme directory like Constructor and Platform, check out the theme-options tag to see all the themes which should have control panels.
Of course there are also commercial themes which have these features as well, like Builder and Prose.