Accordion color should use $accordion-color

[giovannipiller]

This makes it possible to easily override the color without breaking the SASS compilation.

e.g. color-contrast(...) requires a valid color. There are scenarios where it's interesting to pass a CSS variable, which can't be computed by color-contrast.
For example, $accordion-bg: var(--my-accordion-bg) currently breaks the build.

Besides, $accordion-color already exists, so it makes sense to use it.

[giovannipiller]

I'm in no rush, but is there anything that I can double-check to expedite this review?

[mdo]

Sorry for the delay, getting it merged :)