How to plant correctly on a wall made of granite blocks?!

Have now done a little searching. There are really a few nice specimens. Do I just plant the plant in the gaps? The stones are already getting quite dirty when it rains. I hope that the plants will then keep everything away.

Cushion thyme loves the sun and would definitely do great there. Also blooms beautifully and tastes delicious :)

We have now started to place some plants in the gaps. They need to grow properly first. I am still considering planting moss in the horizontal gaps. They are supposed to keep the dirt away.