i don't understand what you mean by "someone elses css".
go to the bootstrap page and look in the dev tools with chrome and inspect all of the .css files that are downloaded on the page load.
then go to your page (without using "someone elses css" code and see what .css files your page is not loading. then when you see what you are missing and include them correctly. if you can't get their css files to work, the you aren't including them correctly or are missing some.
Basically when I started I went to their example page (which must have been a pre 3.0 example) and just took the code and tried to disect it.
I quickly realized this was a big mistake.
So I started from scratch and tried to pull in components. However something as simple as 4 columns of content next to each other would not show up correctly UNLESS I used a css file that was linked in the example. The new bootstrap ver 3 css would not render that as 4 columns.
The solution was to stop using span3.
I have a nice working page now. The only problem I have is the carousel. They have a carousel on their example page but not in the component section explained. I'm not sure if I'm getting grey bars because it doesn't like the image size or for another reason. I've tried to put the whole carousel in a fluid and regular container but the carousel just looks awful. It will not scale correctly and has these grey bars.