Blockly injects invalid CSS

[moniika]

Describe the bug

Blockly injects CSS with syntax errors into page.

To Reproduce

Steps to reproduce the behavior:

1. Open playground
2. Inspect <style> tag with id blockly-common-style and copy/paste content into syntax checker like csslint.net

Additional context

Most of the errors appear to be caused by accidental single quote character being included in css.

Issue noticed when debugging test failure in Blockly Samples RaspberryPiFoundation/blockly-samples#907