Hi Bruce,
good question, never thought about that. Afaik there is no global setting or something you can use.
I have one idea though i would like to share. If you open your developer tools while filling out your form, you can inspect the html that's rendered. You will see that almost all of the tags have Nintex classes assigned. In your SharePoint hive, you will find the css files nintex uses for styling (somewhere like C:Program FilesCommon Filesmicrosoft sharedWeb Server Extensions15TEMPLATELAYOUTSNintexForms hemesase). So you COULD create your own CSS file and change the default border width for all Nintex classes. You would then need to reference your own stylesheet in the form settings of each form.
I haven't tried it myself, but sounds like it could work However, this would be quite complex because you have a lot of html and css classes to check.
If you really try this approach, please keep me informed about how it works. This could not only be useful for border-widths, but in general for default layout settings.
You can also start a new feature-request for your issue as this would be very useful for setting global layout defaults.
Cheers
Philipp