Serious IE7 css issues

i have Serious IE7 issues with my nopcommerce theme implementation and i would like to know how should i fix this?
at the moment i have a html conditional (if IE7) question that adds an additional IE-fix.css style sheet..
but this solution is not that great because the last word of CSS still belongs to the Original Theme folder CSS..
so any shared property i have in both css will overwrite to the original stylesheet and not the IE7-fix one..

basicly what i want to do is to ask programmaticly (in code) which browser is it and by that value change the stylesheet appropriately

One possible solution to your override problem is to use !important in the IE7 specific style sheet.
For example:

.header-logo
{
    margin: 5px 50px 0 0 !important;
}