Expand header-color mixin to articles and blocks

[swashbuck]

Subject of the enhancement

The current .header-color-mixin allows us to add colors to the menu and page header. Now that we can add background images to articles and blocks (#571), it would be useful to expand this mixin to those elements as well.

adapt-contrib-vanilla/less/project/theme-contentObjects.less

Lines 19 to 52 in c9ac1c1

	// Header color mixin
	// Add to menu/page to enable
	// e.g. 'header-color transparent-dark'
	// Note: both arguments must be predefined variables
	// --------------------------------------------------
	.header-color-mixin(black, white);
	.header-color-mixin(background, background-inverted);
	.header-color-mixin(transparent-light, font-color);
	.header-color-mixin(transparent-dark, font-color-inverted);
	.header-color-mixin(@color, @color-inverted) {
	.header-color.@{color} {
	.menu,
	.page {
	&__header {
	background-color: @@color;
	}
	}
	.menu,
	.page {
	&__title,
	&__subtitle,
	&__body,
	&__body a,
	&__instruction,
	&__instruction a {
	color: @@color-inverted;
	}
	}
	}
	}