Shaun, for "Save as Draft" message settings, it sets the message format for the email (after you click save as draft, you will receive an email showing that message and a link to the draft). For now, the thank you message after you click "Save as Draft", it's generated from backend because there would be a link to the draft and currently we don't support customizing the thank you message after save as draft.
If the customer really wants to remove the style from the Thank you page after the "Save as Draft", search "Draft.cshtml" in the forms installation folder (normally it's path/to/forms/Forms/Views/Form/Draft.cshtml), Line 58, delete
class="cf-formwrap"
That is to say, change Line 58
<div id="draft-confirm-box" class="cf-formwrap">
to
<div id="draft-confirm-box">
This will prevent the Thank you message after "Save as Draft" from inheriting the background image set from the form body. Hope this works.