The icons you see on Edwiser RemUI theme are icons pulled from FontAwesome.
However, you can customize the icons being shown by following these steps.

Step 1) Go to font awesome - https://fontawesome.com/v4.7.0/cheatsheet/

Step 2) Download the desired icon

Step 3) Go to Edwiser RemUI theme file > 'pix_plugins' folder > 'mod' folder > inside 'mod' you can choose the activity whose icon you want to change > replace the icon.svg with the desired icon.

Step 4) You need to purge all cache after the changes are done, for that navigate to Site administration > Development > Purge all caches.

The above steps would work if you want to replace the icons with Font awesome icons.

Now if you want to replace the icons with the set of icons available in Boost then you need to follow the steps given below:
Step 1) Go to Edwiser RemUI theme file > 'pix_plugins' folder > 'mod' folder.

Step 2) Delete the 'mod' folder.

Step 3) You need to purge all cache after the changes are done, for that navigate to Site administration > Development > Purge all caches.