fix: make mono theme panel headers black

[murrant]

Fixes issue where the librenms_logo_mono.svg is now loaded by making the background black to match the logo.
Update twofactor.lib.php.
Remove unused if statement, $config['title_image'] is always set.

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[mention-bot]

Thank you for submitting a PR @murrant! We have found the following @arjitc, @Rosiak and @f0o based on the history of these files to review this PR.

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