Implemented light-dark mode theme support.

[luckcolors]

OpenSCAD Playground Pull Request

This issue solves #135 and is a refactor of pull #128.
It is based on the code from user @tekk from said pull request and for this reason i've added them as a co-author.

Changes:

• I have cleaned up the changes to be as minimal as possible.
• Original icons have been kept.
• Github "fork-me" button as been restored.
• Dark theme button has been moved to cog-whell settings menu. By default if no preference is set at application load in userstorage it uses browser theme preference.
• Patched bundler config to include the required files from node_modules.

What kind of change does this Pull Request provide?

Please check the relevant options:

• Bug fix
• Enhancement
• Documentation
• Other (please explain):

How can the changes of the Pull Request be tested?

Application was served using readme.md production commands.

1. The theme button was tested and confirmed to be working
2. The auto theme detection was tested by changing the browser setting and reloading the app.