Looks good !

Some comments :

The 'Graph lines' template only renders for the first page. After that, scrolling in any direction shows a blank background.

The Grid background, in black lines, might be easier on the eye if the lines were rather blue, as the graph lines are

When selecting a template, the Color field under the Preview image offers the possibilty to select a colour, but the background remains white when returning to the sketch.

It might be an idea to include a blank template with the lines and grids, in case a user wants to revert to blank.

Thanks for including Fullscreen option