Add support for custom themes

[thespad]

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• [x] I have read the contributing guideline and understand that I have made the correct modifications

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Description:

Closes #29

Benefits of this PR and context:

How Has This Been Tested?

Source / References:

[LinuxServer-CI]

I am a bot, here are the test results for this PR: https://ci-tests.linuxserver.io/lspipepr/phpmyadmin/5.2.1-pkg-b1edc172-dev-932d42308cb3c7723ce21ab6726e1f0b85bc3308-pr-30/index.html https://ci-tests.linuxserver.io/lspipepr/phpmyadmin/5.2.1-pkg-b1edc172-dev-932d42308cb3c7723ce21ab6726e1f0b85bc3308-pr-30/shellcheck-result.xml	Tag	Passed
amd64-5.2.1-pkg-b1edc172-dev-932d42308cb3c7723ce21ab6726e1f0b85bc3308-pr-30	✅
arm64v8-5.2.1-pkg-b1edc172-dev-932d42308cb3c7723ce21ab6726e1f0b85bc3308-pr-30	✅