Add Flatland to the list of themes

Added the Flatland theme, see below for a screenshot of how it looks

![screenshot of theme](https://github-camo.global.ssl.fastly.net/4547730a054cf3929a656b84c4e23dc5ced4b8e0/687474703a2f2f662e636c2e6c792f6974656d732f32463133334b3267316931553233307a313031522f53637265656e25323053686f74253230323031332d31312d31342532306174253230362e32332e3538253230504d2e706e67)