Update theming.md for angular 6

[cRAN-cg]

Bug, feature request, or proposal:

Proposal

What is the expected behavior?

theming.md should be up to date

What is the current behavior?

theming.md is out of date

What are the steps to reproduce?

Providing a StackBlitz reproduction is the best way to share your issue.

StackBlitz starter: https://goo.gl/wwnhMV

What is the use-case or motivation for changing an existing behavior?

Increase in accessibility and validity of the doc w.r.t the latest version of angular (angular 6 and above)

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Is there anything else we should know?

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}