We have partnered with Codeable.io so that they can fulfill your customization needs. Just fill out the Theme Customization request form and they will get back to you with a quote.
We have developed a simple plugin, called Styleguide, that helps you to modify fonts in all of our themes, and colours in some of them (we plan to support colours in all of them in the future).
If you have knowledge of CSS then the best option is to use the Custom CSS module of Jetpack to make CSS changes to your theme.
We'd love to get some feedback to help us create better themes.