Invite email: color and padding changes

[ethomson]

Summary

Tweak the padding and background color for the email template.

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

Manual email send

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• Included tests that validate the fix or feature.
• Checked that related changes are merged.

[ethomson]

There's a couple of other things that I'd like to change that I didn't here.

• The verbiage should be "become an editor" or "become a member", not "become editor" or "become member". I don't know the best way to effect this - maybe the templating language has conditionals but I didn't want to mess around here.
• The text of the text email. I think it might make more sense to pull that into a template, too, but that's a bit beyond my capabilities.

[coveralls]

coverage: 51.884%. remained the same
when pulling b107213 on ethomson/email
into 5106a3c on main.

[coveralls]

coverage: 51.884%. remained the same
when pulling b107213 on ethomson/email
into 5106a3c on main.