I'm going to be highly biased, in my opinion, since this service is in my wheelhouse. I can only speak to WHMCS and Blesta, but the default theme for both providers leaves much to be desired. There is minimal customization, and the disconnect from your primary site to your billing installation may be jarring.
A custom theme for either billing solution is an intense and often expensive endeavor. I would only recommend it if you're a business seeking a unique identity or value a high degree of consistency in your user experience. It can be valuable but overkill if you do not have a specific business case for it.
Premade themes can be an excellent low-cost option, but they are a dime a dozen. If you're going to invest in purchasing one, it might be worthwhile having it customized to fit your overall look and feel. Before taking the plunge, check your flexibility with the design, colors, fonts, etc.
Another option is a full-site theme. There are a few options here on the WHMCS side, but not so much on the Blesta side. These would give you greater control and allow the billing system to take the place of your site. Take your time and evaluate each one to see if it fits your needs.
I hope this is helpful to anyone in the future.