Are you looking for that specific drop in the ocean? You want to find the perfect Theme for your clients WordPress page? I've done this many times and it always is a time consuming process. There are so many great themes for every purpose and even purpose combinations, you will become a little overwhelmed after the first few hundred themes you've examined. So let us do this in a structured way to at least get the perfect result for all that effort, shall we?
Stem the flood
The first step would be to find lists of recommended themes for your specific purpose. If you need a theme for a WooCommerce-Shop, you might want to look for "best woocommerce themes 2016". You are not interested in last years great themes because web technology is a fast evolving thing and we are planing for the next few years right now, not the past. You can browse these lists fast and memorize themes that get mentioned repeatedly.
Sales | Satisfaction | Support
On to a deeper look into the list of themes that caught your eye. Who is selling the theme. Do they have experience? How did their clients rate the theme? Is there a changelog available with recent entries? Demo page? Support forums? A premium theme deserving the word will have all of the above. We need to keep in mind though, that buying positive reviews is not unheard of.
Another thing to to pay attention to is, if the reviewer gets a commission for selling a theme. It'd make sense to write a positive review then, right? Also if those reviews are controlled by the vendor, it is more likely that they are manipulated. And even if not: review-pimping is a hot topic these days, even at Amazon.
Dissect the frog
If you haven't ruled out a theme yet, it is probably pretty and good with marketing itself. But how does it perform technically? I usually take a look at the demo pages source code. I want to know how many and which resources the theme loads. If it is built on Bootstrap for example, does it include minified versions of the framework? How old is the release of the version included? If a theme comes with a framework from four years ago, it is likely that this is the already outdated version we'll have to live with in the future.
Content is King but Speed is Queen
Your content can be as engaging as it wants, if the user closes the browser in impatience because your site is loading too slow, it doesn't matter. WordPress tends to get slower over time with messy content building up and plugins the client wants installed. So we will make sure that speed wise the initial position isn't bad already. But how can we do that? Simple: ask Uncle Google! Makes sense, as SEO is all about search engines, right?
Sell your conclusion
With this techniques you should be able to identify 3-4 themes that fit the task and perform well technically. Now it is time to confront your client/creative with those. Be comfortable. You can stand your ground now in any discussion. Your arguments are solid, which is much better then just
I like it.