You want your Shopify store to look its best. How that happens is usually involves a jargon-filled wade into the swamp called web development.
There's one well-meaning piece of advice that you should avoid:
Get your store pass W3C validation.
If you don't know, W3C is the non-profit organization that decides how many parts of the web works. This includes the extremely detailed specifications for HTML and CSS that browsers try to comply with.
W3C also hosts a few tools you can use to check any website to see if it confirms to the specification.
I've been doing web development since 2004 and I've never, ever, ever, (ever) seen a real-world website pass the validator without some kind of error or warning. I'm pretty sure most of the main Shopify themes fail too, and that's without apps adding their own code all over the place.
In fact my own website right now can't even be processed by their tool because their tool is making a mistake, throws up its hands, and pouts in the corner.
But my website and millions of others still work.
That's why when a customer asked if they should invest time in fixing their Shopify theme so it passes I told them no.
There are thousands of more valuable things to do other than worry about W3C validation.
A better thing to strive for is a website that works and is visually acceptable in all of the major browsers. That's what really matters to your customers.
If you're getting a new Shopify theme developed, have your developers focus on that and not some silly and impractical standard. You'll save money and they'll save their sanity.
Time for me to put that soapbox away for now...
One valuable improvement that you can make is to get your structured data in order.
Google has been working on Rich Snippets so I bet there's going to be more search enhancements coming out in 2018 for stores with structured data.
And JSON-LD for SEO is the easiest and safest option in the Shopify app store to automatically add that data.
Already trusted by thousands of stores, it's among the top SEO apps overall.
Eric Davis