feature: more sample code for theming material

[captaincaius]

I'm submitting a...

• Bug report
• Feature request
• Documentation issue or request

Current behavior

Having just gone through a big port to material, I thought it might be good to have some commented code come out of the app generator for the following:

• secondary themes (e.g. a dark theme)
• individual theme variable overrides (e.g. for background)
• individual component overrides (e.g. you want to override the toolbar w.r.t. the default)

LMK if it's a welcome addition, and if so, for which branch.

Expected behavior

Minimal reproduction of the problem with instructions

Environment

- generator version: X.Y.Z
- node version: X.Y.Z  <!-- run `node --version` -->
- npm version: X.Y.Z  <!-- run `npm --version` -->
- OS:  <!-- Mac, Linux, Windows -->

Others:

[Viatorus]

Hey, I currently stuck on:

individual component overrides (e.g. you want to override the toolbar w.r.t. the default)

I have a component (like about.component) which should have an extra icon in the header.
What would be the best way of doing this? Just adding the icon to header.component seems totally wrong.

[sinedied]

Sorry for the long delay, this is for sure a welcome addition :) I can be done on a separate branch starting from v6 release as it will (hopefully) merged soon.