feat: add getThemingClass util

[huyenltnguyen]

Checklist:

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)

In order to enable theming, the consumers need to add the theming class (either light-palette or dark-palette) to their body element. This is because some color tokens are defined based on those classes:

• ui/src/colors.css

  Line 92 in 15fbf0a

  .light-palette {

• ui/src/colors.css

  Line 111 in 15fbf0a

  .dark-palette {

I don't really like that the consumer has to spell out the class name exactly as it's easy to make typos, and this also implies that the consumer needs to know the implementation detail (which class names we have).

So, I'm creating a util function to allow the consumers to control the theme while abstracting away the implementation detail.

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Ref:

• Here is how /learn is computing the class: https://github.com/freeCodeCamp/freeCodeCamp/blob/210a0b75dde5f0e1d56796394ae4271c2c62bca2/client/src/components/layouts/default.tsx#L188-L190