Add styling for `:active` button state

[ggeisler]

I don't think we currently provide the user with specific feedback when they've selected a button in the wallscreen UI. I'm sort of guessing since I haven't used the physical screen yet, but I think we want to add some styling to the :active state of the buttons so the user gets a stronger sense that they've "pushed" a button?

(I think worrying about the :hover state might be unnecessary for our physical screen context, but am not positive.)

I haven't tried anything yet but wanted to put this on the radar in case anyone wants to try some ideas before I have time to get to it. I don't think it needs to be anything dramatic, since our application should respond quickly to button selection, but something subtle might improve the user experience when interacting with the buttons.

Note that if I remember correctly @camillevilla posted a video or two showing her interacting with the buttons a couple of weeks ago in Slack so that might be relevant, or @camillevilla might have thoughts on this.

[camillevilla]

Relevant interaction videos are here: https://stanfordlib.slack.com/archives/C02C493RLV7/p1637273314089600

Current behavior

• in kiosk mode, a “press and hold” interaction will apply an :active style to the button, but no action :sunglasses:
• a quick tap will apply :focus grey border and activate the button

I agree that :hover state is not applicable in the physical screen context.