Unable to Save Site as Site Template: Unable to convert the HeaderColorIndexInLightMode XML

Steps taken:

1. Modified a few site pages to separate the text and the images and published.
2. Navigated to site contents > site settings > changed the URL from ending in "settings.aspx" to "savetmpl.aspx" as I have done before from the same site to save it as a site template.
3. Filled up the fields for the file name, etc. Made sure to tick the checkbox to include the content and clicked OK.
4. Site loads and ends up displaying this:
   Unable to convert the HeaderColorIndexInLightMode XML attribute for the WebTemplate XML element for feature {insert GUID} to a positive integer value

Context:

• Also tried to export another site as a site template but same error.
• Tried to import a pre-existing site template (first backup) into a new SharePoint site but same error.
• Double checked and made sure that the following is true before trying to save template again but same error occurred:
  • Custom script has been allowed.
  • SharePoint Server Publishing Feature remained deactivated.
  • No corrupted files in site assets or in the site pages.
  • Content Approval Settings: Require content approval for submitted items is still set to NO.
• Tried using another account with global admin role and site owner full control access to do above steps but did not work as well.

Can somebody please help me on this? I'm not sure what else I could do to be able to save this as a template :(