Fix usability issues in the presentation page

[F0urchette]

Description

• Make the scrolling arrows of the carousel more visible
• Show the students-commits graph image first in the carousel
• Add a scroll button at the bottom of the screen when you reach the presentation page
• Change the color of the green background to the green used for the commits in the overview graph

Hints

For the scrolling arrows, we will try to make them darker to see if they are more visible. For the green background we will use #35C692