Fix invisible button text for hx-indicator in dark mode (#3353)

Without this, in dark mode the [hx-indicator](https://htmx.org/attributes/hx-indicator/) demo button has white text on white background. Switching to `Canvas` means the button background will be black in dark mode.

This is a similar fix to #2719, though we can’t use `primary` here as it would make the indicator "bars.svg" image almost invisible.