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Thanks for the feedback. The Tweak service is not well documented at the moment, and that is something I will improve before the end of the beta phase.
The minicart and buttons are all images, so to change their colour, either load in your own images, or go back to "Themes", make one with the colours you like, save it and use the "copy to CDN option to make it available to your tweak account. You can then select it from Manage > use saved theme.
Any time you click the "Save" button your Tweaks will be saved to the active Tweak (by default this is named "current". I think you may have created a new Tweak from the "manage" tab, but the previous one should still be there, probably called "current" and you may be able to select it from the "load different tweak" drop-down.
I hope to add more tips, help and documentation soon. I know it is a bit confusing right now.