Cache themes and retrieve from cache when available

[banani720]

Description

This pull request caches the theme data with an expiry date of 30 days to local storage. If a theme exists it is returned and no call to the CDN to get the theme are made

Closes #85

What change does this PR introduce?

Please select the relevant option(s).

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation update (changes to docs/code comments)

What is the proposed approach?

To resolve this issue I simply added cache setting/getting functions in order to check for a cached theme before going to the CDN. If there is no cached theme or it is expired, a call to the CDN is made and the result is then cached for future use. I chose 30 days as an arbitrary number but we can expand or reduce that as needed

Checklist:

• [x] The commit message follows our adopted guidelines
• [x] Testing has been done for the change(s) added (for bug fixes/features)
• [x] Relevant comments/docs have been added/updated (for bug fixes/features)