fix: prevent unnecessary themeColor deprecation notice

[jhildenbiddle]

Summary

The following deprecation warning was spamming the console even when $docsify.themeColor was not being used:

$docsify.themeColor is deprecated. Use a --theme-color property in your style sheet. Example:
<style>
  :root {
    --theme-color: deeppink;
  }
</style>

This console message will now only appear when $docsify.themeColor is used.

Related issue, if any:

None

What kind of change does this PR introduce?

Bugfix

For any code change,

N/A

Does this PR introduce a breaking change?

No

Tested in the following browsers:

• Chrome
• Firefox
• Safari

[vercel]

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Name	Status	Preview	Updated (UTC)
docsify-preview	✅ Ready (Inspect)	Visit Preview	Apr 11, 2024 3:08pm

[Koooooo-7]

LGTM.