We’re looking at using the Contribute module as it works the way we want rather than Collecting, but its appearance under the default theme leaves a lot to be desired with the widgets looking nothing like they do under admin.
Are there themes that look better, or do I need to copy in some of the admin theme for the widgets?
Yes, you can style the public pages of the module as any other public pages. Furtheremore, there is a specific theme that was done for some french universities (see https://dante.univ-tlse2.fr/ or https://utheme.univ-tlse3.fr/).
The main difference is the fact that resource templates are used to build your forms.