Mix of p-1 and hardcoded values used to create activity chooser grid

The new activity chooser has a grid of options.

To get 6 across, the CSS is setting their width to 1/6 of the width after subtracting .5rem. The .5rem is to account for the two p-1 used on each side (which is 2.5rem in the default theme)

If a theme changes things such that the p-1 is no longer exactly equal to .25rem, then the grid breaks, and you get a gap at the side when looking at all the activities.