I have 5k CSS lines for Firefox, Opera and sometimes IE 8. After all this, the account told me that it should work correctly in IE 7. How can I do this without rewriting 40% of the code
After going through and fixing the obvious problems, try the following: http://code.google.com/p/ie7-js/
This is not a magic bullet, and it can cause problems. Personally, I simply agree that time will be taken on the site, and during this time I must draw up reports that I must make for the bosses.
If you start working with IE8 and make money on this site, it will be easier to switch to IE7, which will facilitate the transition to IE6. Note that in IE8 there are some debugging tools that do not fully suck, be grateful to them.
http://websitetips.com/css/solutions/
http://www.webcredible.co.uk/user-friendly-resources/css/internet-explorer.shtml
http://css-tricks.com/ie-css-bugs-thatll-get-you-every-time/