I have to say I am disappointed as I was hoping to be able to truly modify the theme and not just make the same setting adjustments available under the themes now.
For example, it would be very helpful to be able to adjust the size of the icons/text, adjust the padding of the buttons and background colour of the block behind these elements.
To me, the whole point of being able to conveniently modify a theme is for these reasons - not just another way to make the same setting changes available in the component already.
I have a site using the default theme. When viewing on mobile phone, the individual attachment's container go off the right side of the screen - here would be a perfect example of being to modify a theme template under the plugin (not custom CSS) to reduce the icon and text and adjust the left/right padding of the download button in order to make the container less wide and hopefully better fit the viewport.
I hope you consider this