Bring some style to the admin interface

I see 2 methods:

• coding from scratch as SCSS (should not be too long)
• importing a minimal framework and scoping it into an ID, so that it does NOT conflict with the client's page

#teklia-virtual-keyboard {
  @import "path/to/frameworl.scss":
}

Design wise, you can "steal" the look from Bulma modal: https://bulma.io/documentation/components/modal/