Add WebIDE Dark Theme Support

Problem

In #195701 (closed) Dark syntax highlighting preference for the code editing area was introduced in the Web IDE. However, this doesn't extend to the rest of the Web IDE for a complete dark experience.

Proposal

Add support for the dark theme following Version 1 of the designs, which is available here https://www.figma.com/file/1dJYV424qiSDVdmS5BKsNz/Dark-Web-IDE?node-id=93%3A218

Acceptance Criteria

• Theme can be enabled
• Theme can be disabled
• Theme matches the Design

Technical Implementation Details ... TBA