[h2] center, add more padding, use darker grey and increase size

[smkraus]

We'd probably need to create a different class because h2 is also used on the cover.

• I'd suggest we text-align: center it (same thing goes for any text and all elements on the site).
• at least 20px more padding below and 50px on top (depending on padding of container); problem: "what we do"-section already has a lot of padding/margin from somewhere else
• I'd suggest we use a darker grey like #645D56
• I would also love an increased size like 3.5 em (if that doesn't create too many problems for mobile)