the amount of work to widen a theme really depends on the way the original theme is structured. some are much easier to widen than others – ranging from changing one width declaration in style.css to digging through many graphic background images and adjusting dozens of lines of css.

so, the first step could be to post the name of your theme, and your ideas of an ideal width.
or, even, better, post a link to your site.
the more information you give, the more help you might get.

although, helping with pure css issues is not primarily the purpose of this forum, you’ll probably get some pointers – even if it is links to online resources of ‘learning’ css, or to the respective style declarations.
help given is often aimed at pointing you to online resources which you can use to learn about the topic yourself, so that you grow in understanding the programs, php, html, and css. and get indepenant of asking questions for every single problem.

similar to the folowing proverb:

Give a man a fish and he will eat for a day. Teach a man to fish and he will eat for the rest of his life.
-Chinese Proverb

it is actually not a ‘massive amount of CSS’ you need to learn, more a tedious working through the css file line-by-line and changing the width value of the corresponding styles by the same amount.

and there are free tools, such as firefox web eveloper add-on, or firebug, that make the understanding of the css structure a bit easier.