Implement backend support for theming

[DL6ER]

By submitting this pull request, I confirm the following:

• I have read and understood the contributors guide, as well as this entire template.
• I have made only one major change in my proposed changes.
• I have commented my proposed changes within the code.
• I have tested my proposed changes, and have included unit tests where possible.
• I am willing to help maintain this change if there are issues with it later.
• I give this submission freely and claim no ownership.
• It is compatible with the EUPL 1.2 license
• I have squashed any insignificant commits. (git rebase)

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What does this PR aim to accomplish?:

Add support for pi-hole/web#1253

How does this PR accomplish the above?:

Add pihole -a theme option.

What documentation changes (if any) are needed to support this PR?:

None

Note that the target branch needs to be changed from master to devel once #3324 is merged.

[PromoFaux]

Oh dear. Rebase on development and submit against the development branch please ;)

[DL6ER]

@PromoFaux Well, see my last sentence in the description - I missed it. Done now.

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/pi-hole-5-1-released/35577/1