WordPress Version

Hi everyone,

I just started a new website project and took over an existing WordPress website.
No that I wanted to get started I logged into the WordPress backend and saw that the backend looks completely different from the other sites I worked on before.
It is in a light grey and says that version 3.6.1 is the current one.

My other websites have a dark grey backend and a newer version of WordPress.

Does anyone know what the problem could be here?

Thank you,
Anna