Hi there,
When I look at your Custom Design Upgrade page I see three tabs, one for font, one for colors, and one for CSS. The CSS tab is where you can make changes to your theme's stylesheet code. If you aren't seeing this tab, could you take a screenshot that shows me what you do see and upload it to your Media Library? If you leave a link to it here I'll be able to see it.
http://en.support.wordpress.com/make-a-screenshot/
It's true that you see the same options here before and after you purchase the upgrade. However, prior to purchasing the upgrade any changes you make are preview only and cannot be saved. The ability to make your font, color, and CSS changes 'stick' is added once you purchase the upgrade.
In regards to not being able to access the signup page, could you try accessing it in a different browser and let me know if the blank page problem persists?
Thanks!