fix dark mode being applied to non-pdf page

[NShreedesh]

Description

Currently, the dark mode is being applied to non-pdf pages. This Pull Request updates dark mode to be applied only on pdf files.

Type of change

• [x] Bug Fix (non-breaking change which fixes an issue)

How Has This Been Tested?

By testing in different pdf files

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] My changes generate no new warnings