Add SASS variables for message background colors

[aboutthomas]

This is about Bulma.

Overview of the problem

This is about the Bulma CSS framework.
This is a Sass issue: I'm using version 0.7.2.
I am sure this issue is not a duplicate.

Description

I'd like to access the message background colors such as

bulma/css/bulma.css

Lines 5899 to 5901 in 3191745

	.message.is-warning {
	background-color: #fffdf5;
	}

via SASS variables. Not sure what the right naming would be, maybe $warning-light or $warning-background.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.