update theme directory

```
2019-10-06 10:35:14 [WARN] (mdbook::renderer::html_handlebars::hbs_renderer): Previous versions of mdBook erroneously accepted `./src/theme` as an automatic theme directory
2019-10-06 10:35:14 [WARN] (mdbook::renderer::html_handlebars::hbs_renderer): Please move your theme files to `./theme` for them to continue being used
```