Code errors

I have recently built a website using the vantage theme and had an SEO audit completed, which uncovered lots of HTML and CSS errors in the code generated by the theme, hindering the site from performing well in terms of SEO. Is this a common problem with an obvious solution? I am relatively new to web design and do not want to tamper with any HTML or CSS generated through WordPress that I don’t completely understand!

URL: http://www.yourprincessparties.co.uk