I’m not sure if this is a bug or a mistake in the docs but if you create a new theme and only provide an “override.css” file the control panel looses all its imagery. To get the images back you need to copy the “images” folder over from default to your theme. This seems to contradict:
ExpressionEngine will attempt to load an override.css stylesheet that, if present, allows for easy customization of styling. For example, if you just want to change the color of some elements, your theme might consist of only one file.