Default File Associations

Setting default programmes is actually very simple and straight-forward.
Open your user's defaults.list.

vim ~/.local/share/applications/defaults.list

Add lines formatted like this:

<mimetype>=<desktop file>

Here are some examples from my file.

text/html=sublime_text.desktop 

text/plain=sublime_text.desktop

x-scheme-handler/http=google-chrome.desktop

x-scheme-handler/magnet=transmission-gtk.desktop

The changes will be applied immediately.