Great suggestion. Learning design software, at least basics, can be productive.

Some designers will disagree on this point—many are moving away from Photoshop altogether. However it's more difficult to try color variations for a gradient by typing in hex colors, for example. Including up some basic techniques would make sense. (Every professional designer has a few go-to tricks.)

I'm thinking about providing some free design files with the book. Just basic elements, like buttons, styles, etc than can be adaptable. (And easy to translate into CSS.) Would that be helpful for you?