This should do the trick:
ul { list-style-type: disc }
ul ul { list-style-type: hyphen }
The above CSS rules specify that a disc (bullet) should be used for list items, but a list within a list should use a hyphen instead.
However, you may find that Prince insists on using the UNICODE hyphen character (U+2010) that does not exist in all fonts, leading to the second rule producing question marks instead of hyphens. A workaround is to specify the second rule like this:
ul ul li::marker { content: "-" }
This rule directly specifies the content of the list item marker and can be used to place any text there at all. (Perhaps it would be useful if any text could be used for the list-style-type property as well, something for us to consider).
Another point worth mentioning is that in some situations it can be more convenient to select the list using an explicit class name rather than using descendent selectors, to make it more obvious what is going on:
ul.bullet { list-style-type: disc }
ul.hyphen > li::marker { content: "-" }
<ul class="bullet">...