If you want to change that, you need to know a bit HTML and CSS.
It seems the view file is in somewhere here:

"oxwall\ow_plugins\notifications\views\components\notification_html.html"

Looks like it is in the languages section, but it's in different keys instead of just one.  For example; the top section where it says Dear username Here's the latest activity... is in a key by itself.
Note that emails have a text version, and an html version (i.e. you will see the same thing twice, but in the key you will see txt, or html).