Evaluate and update appearance for contrast

[jbphet]

I did a quick demo of this sim in its current incarnation during a status meeting on 12/19/2024. @terracoda expressed some concern about the level of contrast in the visual design, since the sim currently had a light-colored background and a number of light-colored design elements. We should make sure we evaluate this and update the visual design as appropriate.

[jbphet]

In the status meeting mentioned above, @kathy-phet mentioned that there are some WCAG guidelines for contrast. While these mostly discuss text, they might still help to inform our decisions on this. Here's a link: https://www.w3.org/WAI/WCAG21/Understanding/contrast-minimum.html.

[jbphet]

@terracoda - Do you have any thoughts to add about how we should approach this?

[jbphet]

Over Slack today @kathy-phet said:

Quick note: Particles are probably too small to pass contrast work now, so should revisit the size.

[jbphet]

Adding @arouinfar as an assignee, since I'm not sure what should be done next from the developer perspective. Shall I set up a meeting with stakeholders? Is there some reasonably short guidelines that I should read through and try to apply?