Why do the styles look broken in Presentation Mode?
Make sure you dont have options mixins applied in the _theme.scss file while in Presentation Mode or you will have CSS conflicts. Comment them out or remove them until you are ready to preview the applied mixin styles.
Why are all the options resetting when I load the Checkout view?
There is a current limitation in the application that prevents retaining the theme settings when you load the Checkout page. You should only load the Checkout page view in either Preview Mode during development or before setting any config options. Do not load the Checkout page in Presentation mode if you have configured any theme options or you will lose the settings. You will not be able to preview your options applied to the checkout page in Presentation mode at this time.
What options are configurable outside of what is available in the Theme Designer UI?
You can see a full list of the configuration options for each mixin, as well as the description of what they do on the Mixins tab of these instructions.
Why am I not seeing the styles applied when I add them to the _theme.scss file?
First, make sure you have Preview Mode enabled. Also make sure that you have updated the CSS imports in the checkout.vue and default.vue files to point at the client theme you are working on. By default this is set to load the Clean theme.