Color and layout fixes and theming simplification.

[xorgy]

• Always include the expander in the state tree layout so that keys at the same level always line up.
• Always apply a theme class to to simplify theming.
• Remove all app.css theme overrides.
• Move all single-use PostCSS variable values to the reference site.
• Remove all unused PostCSS variables.
• Correct tab menu light theme styles.
• Move the Router Tree outside of the , which belongs to the Component Tree.
• Remove unused node-item styles.
• Move some component-specific styles from base.css to appropriate component css files.
• Fix light theme colours.
• Use ViewChild and element refs in Header instead of querySelector and global ids.
• Wrap theme selector menu radio labels around their corresponding radio inputs so that clicking them selects the corresponding radio button.
• Remove unnecessary null check on event.target in Header.resetIfSettingOpened(), since every onclick event has a non-null target element.
• Use .transparent instead of .invisible on non-expandable state editor nodes, to prevent mouse events from being emitted.

[mention-bot]

@xorgy, thanks for your PR! By analyzing the annotation information on this pull request, we identified @sumitarora, @clbond and @danielschifano to be potential reviewers

[sumitarora]

@xorgy UI changes looks good few things Show HTML elements is not working

[xorgy]

@sumitarora strangely, the CI CRX build seems to be of an older version. There may be some state which is preventing it from building the new version.

[xorgy]

@sumitarora I just tested the CI CRX from this, and it is functioning as expected. The Show HTML elements toggle is working as well.

[igor-ka]

@xorgy LGTM