Switch onion connection icons to use --icon-color-critical and --icon-color

When the user is "potentially blocked" (bootstrap failed) we show a red slash in the onion icon shown in the application title bar and in about:preferences#connection.

Currently the red color is hard-coded, but we could switch it to --icon-color-critical. At the moment, this would mean switching from

• #c50042 to #cf1748 (actually an oklch color) for light theme.
• #ff9aa2 to #ffa0aa (actually an oklch color) for dark theme.
• The standard icon color for high/low contrast themes.

Also, I wonder if we should change the rest of the icon from currentColor (text color) to --icon-color. So from

• #15141a to #5b5b66 for light theme.
• No change for dark theme.

@felicia @donuts are you ok with this?