Add a Backstage UI version of the Aperture theme

[awanlin]

The Demo site currently has an example theme called Aperture. We should create a Backstage UI (BUI) version of this theme. This will help adopters see a working example of BUI custom themes in action.

Details on creating a BUI theme are here: https://backstage.io/docs/conf/user-interface/#create-a-theme-for-backstage-ui-new

The current Aperture theme code is here: https://github.com/backstage/demo/blob/master/packages/app/src/theme/aperture.ts