The " W3CMarkup Validation" site is not helpful for evaluating a theme. It may help you determine problems with "markup" which is related to metadata for structured data such as OG but it can only evaluate your use of markup, not related to the theme. There is a new post here about the value of structured data: [
webmasterworld.com...]
You can validate (free) the HTML and CSS at ht
tps://jigsaw.w3.org/css-validator/ and you can get a report on any errors or things to fix. That does not evaluate the templates in a theme, only their output - the visible page that results. Validation is nice and can be helpful but it has little to do with the quality of the theme.
MANY WP themes have issues, even premium ones, but knowing the problems can allow you to fix some of them. Browsers are very forgiving on most issues and will figure things out usually so I would not necessarily use it for decision making on themes.
As for recommendations - we don't do recommendations or reviews of themes here because they are quickly filled up with drive-by promos. The WP Themes gallery lists thousands of paid and free themes, many with specialty features for specific types of sites. It takes a little time, but they offer filters so you can choose which features are important to you. Their user reviews are curated and more reliable than most theme reviews.
It helps to visit the theme's page and check out their support section to get deep information as to how long they have been around and how well they respond to support requests. You can check that themes are validated with the most recent update to WP and see how users like their features. Definitely, some are easier to use than others as you will notice in their support responses.