When using the Single-line comment form, you'll be presented with a unstyled list of comments and commenting form - not ideal. This is however intentional, as we don't believe we should force any style or functionality on your use of Comments.
The HTML code generated by the Single-line comment form tag is based largely on Bootstrap, and is easily stylable through the many classes provided. Have a look at comments.html for the templating code used.
If you wish to customise this form, consult the Template Overrides section.
Available for Craft 2 for free, Craft 3 coming soon for $49.