5000 lines of CSS? You really need to re look at how you have used CSS. I can not imagine any web application needing that many lines of CSS.
The reason we have 5000 lines of well-organized CSS instead of 10000 lines of poorly-organized CSS is thanks to some heavy refactoring that we have done after most of the pages had been built. Don't underestimate the cost of building a unique brand. We can't all get by on 37signals style.
WTH does this mean in this context? Not being snarky; I can't figure it out for the life of me.
However in our case we had a graphic designer who had never done anything on the web before. This may seem like an insane way to design a web product, but the result is that we ended up with a brand that was entirely unique on the web, and this gave us a real tangible benefit when it came to attracting our initial userbase and business partnerships.
In the end the same functionality could have been built and reasonably well styled with 1000 lines of CSS, but we purposefully chose to go the longer route for branding purposes.