refactor(theme): improve custom editor theme api

[riccardoperra]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/riccardoperra/codeimage/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[X] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #58

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[ ] No

Other information

[changeset-bot]

🦋 Changeset detected

Latest commit: e3ca614

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
@codeimage/app	Minor
@codeimage/theme	Minor
@codeimage/config	Major
@codeimage/ui	Minor

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/riccardoperra/codeimage/8KEMudzdDgiLMRYdNTCfwEhRLecw
✅ Preview: https://codeimage-git-212-theme-api-improvements-riccardoperra.vercel.app

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Updated
codeimage	✅ Ready (Inspect)	Visit Preview	May 4, 2022 at 7:32PM (UTC)