SEO Wordpress template and code fixes

I have a Wordpress website that requires some fixing. I primarly need somebody who knows Wordpress, coding skills, and SEO skills for what is required to check if the site is coded correctly for optimal SEO. 1)The primary issue is that it has been identified that template or theme has some bad coding, and that it needs to be cleaned up. I had a previous web developer in Freelancer create a new templet or theme and my SEO guy is saying that the code is very bad and affecting the rankings for Google. The site I have was ranking very high in Google before but has fallen since. Only in Yahoo and Bing is it still very high. I will provide you basic instructions from my SEO person on what he is asking to fix with the template. The website also has some HTML and CSS Validation error issues, about 175 that I would like you to review the coding to help clean this back code from hurting my SEO rankings. I know this can not all be cleaned but I want it to be reviewed for possible problems and fixed. This site is very well optimized so all precautions must be taken not to modify content or SEO tags and metas. I will need you to just fix coding. I also need some very small fixes. 2) Checkout page ([login to view URL]). Make "Proceed to checkout" button GREEN similar green to color under cart heading. 3) fix featured image size in blog posts to all be the same size on the main page to the small size that you will see from the old posts. Good luck with you bidding, and please don't bid unless if you know what you are doing. I had a previous freelancer create damage, and I don't want that to happen again so only serious developers please. Thank you.