Indicate that disabling a theme also resets its settings

[klonos]

Follow-up to #3478

@quicksketch:

Seems like we can merge this as-is, but we need a follow-up to indicate that disabling a theme also resets its settings.

@docwilmot:

This PR makes disabling a theme a destructive process now. For a complex theme that would lose all configuration, this may be unexpected. Should we add a confirm form to theme settings form?

Here it is 😉

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PR backdrop/backdrop#2507

[herbdool]

I created a PR backdrop/backdrop#2507. I took @docwilmot's approach of adding a confirmation form when disabling a theme.

[klonos]

Thanks @herbdool 👍 ...PR looks good. I have left some comments.

[klonos]

This is RTBC ...I will file a separate issue about the things mentioned in the PR comments.

[quicksketch]

Looks good! Merged backdrop/backdrop#2507 into 1.x and 1.12.x. Thanks @herbdool and @klonos!