[Feedback] Consider a filled icon treatment for hollow icons

[ericwbailey]

Note: Moved to https://github.com/github/primer/issues/1303

Describe the topic

This issue arose from a conversation in https://github.com/github/accessibility-audits/issues/477.

The gist is that we are currently relying on only color to communicate state, which runs afoul of WCAG success criterion Understanding Success Criterion 1.4.1: Use of Color.

One proposal for addressing the issue was to use a filled treatment for the icons.

Example from Reddit's up/downvote functionality, as provided by @stephaniegiang:

This solution is ideal in that it provides a visually obvious indication of a pressed/active state. Compare this to a bolded text treatment of the vote count, which:

• Might be too subtle for some people with low vision, and
• Shifts the width of the UI due to the thicker letterforms.

Additionally, a filled treatment significantly reduces the complexity of managing color contrast requirements for state.

While filled arrows would help with this issue in particular, I feel filled treatments would also be useful to have for many Octicons for similar usability issues, both current and future.

[ericwbailey]

Closing to move the issue over to https://github.com/github/primer/issues/1303.