Take the top 20 admin themes from Themeforest and compare them to the UI/UX of 20 randomly selected YC companies. Not the front page; the actual UI.
Reasonable people will disagree which one wins.
But what's crazy about that is that the Themeforest stuff costs twelve dollars†, and is available instantly.
Desktop software developers don't have inferiority complexes about how samey their apps look. In fact, refined sameyness is a point of pride among Cocoa devs! But for some reason, every web app needs to be designed by Shepard Fairey and Khoi Vinh to launch.
By all means, contract Fairey and Vinh. But do it after you launch the product and start making money. Your initial users do not give a fuck about this issue. If you are not a natural, fluent designer, you will lose tens of thousands of dollars of your time (and, worse, money) just so you don't look like someone else's CMS skin... which none of your real customers will notice.
† Admittedly more if you need an all-purpose license, but still 1/10th what a designer costs.