Unless it's key to your target market - if the USP of your product relates to excellent design - it can wait.
If you're starting with something bootstrap based, and haven't mauled the standard templates too badly, you're already at not-ugly. Which beats most folk.
Concentrate on validating your product and market first. Spending too much time on visual design and visual branding at this point can be a mistake.
The sooner you start getting customers the sooner you start getting feedback. The sooner you start getting feedback the sooner you start understanding what works and doesn't work. The sooner you understand what does and doesn't work the sooner you can focus on changing your product to better meet your customer needs.
If you spend a lot of time on visual design now - you may find you have wasted a lot of time and money polishing something that nobody wants.
Validate that people want your product first.