guide(theming): update custom theme section

[fxck]

reasoning in this issue #1786

[willshowell]

@fxck@jelbourn I strongly agree with this change. I lost more time than I'd like to admit tracking down why my bundles kept increasing in size, only to realize the theming guide was a slightly misleading.

[jelbourn]

I've opened #4021 to try to address both this and a couple of other common issues. Let me know what you think.

[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.}