take first theme in list if chosen theme is not in list
fixes #15 Bfore applying the theme, it checks if it actually exists in the list of available themes. If not, it takes the first theme available (in our case, by alphabetical order, Acario).
[A venir] Mise en place de quotas sur nos instances GitLab. Plus d'information sur : https://doc-si.inria.fr/pages/viewpage.action?pageId=129728565
[Coming soon] Quotas installation on our GitLab instances. More information: https://doc-si.inria.fr/pages/viewpage.action?pageId=129728565
fixes #15 Bfore applying the theme, it checks if it actually exists in the list of available themes. If not, it takes the first theme available (in our case, by alphabetical order, Acario).