Draft: Test solarized dark theme for code blocks

Related to #990 (closed)

Use accessible solarized dark theme for code blocks.

• Before: https://docs.gitlab.com/ee/development/documentation/styleguide/#code-blocks
• After: http://accessible-code-blocks-dark.178.62.207.141.nip.io/ee/development/documentation/styleguide/#code-blocks
• Before: https://docs.gitlab.com/ee/development/testing_guide/best_practices.html
• After: http://accessible-code-blocks-dark.178.62.207.141.nip.io/ee/development/testing_guide/best_practices.html