Can't customize header image (logo) anymore

Hi!

Here’s my blog : http://noussommeslespopulaires.com/

The theme is “Sight” by WPShower, and it’s been updated in the last year or so, and it seems I cannot modify my logo (header image) anymore…

The last time I updated my logo was in September 2013.
It used to be pretty simple (Settings > General, I think).

Also I cannot access the stylesheet or header.php anymore…
There’s nothing in the customizer concerning the header…

I’m lost! 🙁

Do any of you guys know of a way to bypass that? A plugin maybe?

WPShower are not replying to my e-mails.

Thank you!

Sam