If i made a site and site looking well in all a grade browsers but if site shows some errors in both XHTML and CSS validation then is there any benefit to solve those errors?
Is site with 10 validation errors better than site with 35 validation errors or both are same for search engine?
Does search engine read css file? Does CSS validation has any importance for SEO?
That depends on the kind and place of errors, I guess.
If these make your site semantic structure undecipherable then, yes, sure they will be a problem for any automatic analysis tool.
If these are non-standard attributes (or incorrect attribute values) then they won't.
Lets use Google for an example...
Firstly, Googles homepage isn't even valid they purposely don't close quite a few of their tags to save on bandwidth. There are also lots of custom attributes.
Secondly, they don't read CSS for SEO. The only time they do is for the caching feature alot of the popular ones have. Google doesn't read the CSS file it just links to it however Yahoo caches everything (CSS, Images, ect...) but that's not going to effect SEO.
And there are lots of good reasons why you shouldn't worry about validating your site. Its good to check to get rid of any huge errors however as long as it works in most browsers you are find. Search engines just care about your content not the way it looks.
most screenreaders are based on the w3c standards, aswell as usability stuff. When you'll follow the w3c standards there's a better chance that the screenreaders will be able to understand your content and display it properly. It's not that you need to put a lot of extra work in making your sites W3c valid so I would just do it. It really depends if you find it important or not