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.

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