The PeopleHR background appears when the vacancy from your website is linked directly to the application form, and not the page of the ATS vacancy.
If you wish to redirect applicants directly to the application form and change this background, this is picked up from your company logo.
To change your logo:
Click Access Button
then click HR Admin.
Click Settings, and then click the Company tab.
Upload your company logo or a blank white image and this will be picked up on the RSS feed.