Sorry but version 3.7+ no longer includes the 'Mini' theme. Properly maintaining two different themes was just too much work, especially as there wasn't very much difference on mobiles. The default theme will continue.
Introduced August 2020
The 'Mini' theme is shown here »
Included in superMicro CMS Version 3.1 (August 2020). To activate the 'Mini' theme, go to Admin: pages » Styles: » Select a stylesheet: and select 'Mini styles'. Then Get styles / Update styles.
Note: the maximum content width of the 'Mini' theme is 640 pixels. Bear this in mind, especially when switching from the default 'White' theme (which has a content width of 740 pixels) and you have images that are 740 pixels wide. They will be shrunk to the 640 pixel wide column.
Restoring the default theme
To switch back to the default 'White' theme, repeat the process above, selecting either 'White styles' or 'Default styles'. They are normally the same except that /css/default.css is minimised. If you have edited the (always) active stylesheet /css/stylesheet.css either copy and save the styles somewhere – or do a backup – then reinstate it.