Stylistic changes to visual representation of injector graph and router tree

[hilaryhacksel]

Resolves #621

• Fixed arrow marker colour
• Fixed root node stroke and fill
• Changed dark and light mode blue colour so that it stands out agains black background
• Introduced new colour variables

[mention-bot]

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

[igor-ka]

@hilaryhacksel LGTM