Use user_application_theme helper instead of hard-coded theme classes

What does this MR do?

Makes use of the user_application_theme helper method to return the proper UI theme CSS class name to use rather than relying on a hard-coded value (such as ui-indigo).

This fixes a bug where these pages will render correctly for an unauthenticated person (because we don’t have a user-specific preference to load) and for a user with the default theme (indigo) set, but the pages will render without a nav-bar background color for all other users:

Screenshots

With a theme preference other than the default set, ui-light-indigo in this case:

Before	After

Does this MR meet the acceptance criteria?

Conformity

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  • I have included a changelog entry.
  • I have not included a changelog entry because this is a developer facing change.
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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
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• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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