Changes the theme path root to be the web root.

Let me explain my situation:

I'm using Bolt thought composer and I have my web root in a `web` directory and not in my project root. To make the themes work before, I changed the `theme_path` in the config file to be `/web/theme`. It was working fine, until today when I decided to update Bolt. After this the `{{ paths.theme }}` in my Twig templates was returning `/web/theme/my-theme/`, and because the web-server doc root is point to `/web` I was getting 404 for those assets in my theme directory. After checking my options, I decided to change back the `theme_path` in the config to use the default one (`theme`). But then, the Twig could not find the template files, since it was trying to look into `/theme`, and it should look into `/web/theme`, hence my PR. 
If you think there is a better way so solve my problem without the PR, I will appreciate it. In any case, at least I provided a solution that works for me.