Are the Sandbox and Toolbox themes only customizable if you purchase the Custom Design Upgrade? It would seem, by the nature of the themes, that shouldn't be the case, but it doesn't explicitly say either way on either theme's info page. Toolbox's info page implies that it would be optimized with the upgrade, not that it would be unusable without the upgrade. Sandbox doesn't mention the upgrade at all.
Unfortunately, as I'm just starting out with this, I don't have the $$ to spare on upgrades and premium themes (no matter how much I want to play with CSS), so if the upgrade is required to make those themes usable I'll just pick a different theme and move on :-)
