Deprecate theme attribute in site config JSON

[itsyme]

What is the purpose of this pull request?

• Documentation update
• Bug fix
• Feature addition or enhancement
• Code maintenance
• DevOps
• Improve developer experience
• Others, please explain:

Overview of changes:
Resolves #2232. Fully deprecates theme attribute in site config JSON.

theme attribute in site config JSON has been fully deprecated.

To migrate:
Use "style": {"bootstrapTheme": "..."} instead of "theme": "...".

For example,
Original (in site.json):

{
"theme": "bootswatch-cerulean"
}

Updated (in site.json):

{
  "style": {
    "bootstrapTheme": "bootswatch-cerulean"
  }
}

Anything you'd like to highlight/discuss:

Testing instructions:

Proposed commit message: (wrap lines at 72 characters)
Deprecate theme attribute in site config JSON

Theme attribute in site config JSON is supported, though not
documented.

This could be confusing for users when trying to search for the
theme attribute in documentation or when referring to older
MarkBind sites which still use the theme attribute.

Let's deprecate the theme attribute in site config JSON fully so that
all MarkBind sites use style.bootstrapTheme, which is documented.

This prevents confusion for users by standardising the use of
style.bootstrapTheme to set the bootstrap theme.

---

Checklist: ☑️

• Updated the documentation for feature additions and enhancements
• Added tests for bug fixes or features
• Linked all related issues
• No unrelated changes

[yucheng11122017]

LGTM! Thank you @itsyme

[lhw-1]

Thanks for the PR @itsyme! LGTM 👍

Are there no tests that need to be updated for this?

[itsyme]

Thanks for the PR @itsyme! LGTM 👍

Are there no tests that need to be updated for this?

I was worried about that as well, but I searched through all instances of theme, style.bootstrapTheme and looked through the Site.test.js but found nothing relevant to any tests to be deprecated.

Though I found something else in packages/core/src/Site/index.js but am not sure if we want to deprecate it this way or change the variable name to bootstrapTheme instead.

[itsyme]

Wondering if we should rename the theme to bootstrapTheme?

[raysonkoh]

I think we should rename it. Makes more sense since the function is called copyBootstrapTheme.

[raysonkoh]

LGTM other than the minor naming issue.

[jovyntls]

LGTM! Thanks for the quick fix @itsyme

Could you update the commit message before merging? Reasoning being that this is a breaking change, so we may want more context in the commit message for future reference.