Hi,
The background etc comes from the template selected in the notification emails in System>Emails. By default, it's the "default" template which is selected, which is the same as for all the other emails sent by HikaShop for users.
And I do not see a problem with these reminder emails display.
I'm thinking the issue might be with your setup, and not with the emails by themselves.
Could you please check their settings in System>Emails ? Could you provide screenshots of their settings and some screenshots to illustrate the issues that you have with their look ?