Simplify theme handling

[DL6ER]

What does this implement/fix?

Remove theme.lua, source everything there is to know about the themes through Lua instead - pihole.webtheme() does now return a full table having all the details.

Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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Checklist:

• The code change is tested and works locally.
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