Translation docs/tutorial/using-a-theme.md

[qlawmarq]

docs/tutorial/using-a-theme.md を翻訳しました。
お手数ですがレビューお願い致します。

原文ページ：
https://www.gatsbyjs.org/tutorial/using-a-theme/

[hirotaka]

ありがとうございます！すごくわかりやすいです。
全体的に半角全角の前後にスペースをいれさせていただいたのと、いくつかsuggentionしたので、ご確認お願いします 🙏

[qlawmarq]

@hirotaka
ご丁寧にレビューしていただき、ありがとうございます！
修正を取り込みコミットいたしました。

[uetchy]

🎉

[hirotaka]

LGTM 👍

[gatsbot]

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