1. install browser plugin/addon 'stylish'.
2. create a new style in stylish
3. copy your elgg theme's css file into the new style.
4. remove any php from the css file
5. replace any local urls with absolute urls.
6. add the following lines to the top of the text file (style):

@namespace url(http://www.w3.org/1999/xhtml);
@-moz-document domain("community.elgg.org") {

7. ad a closing curly bracket to the end:
}

8. hit save and view community.elgg.org - you should now see most of your theme's elements being used.
9. where elements are not being used you may need to add !important to the end of each unused style declaration.