Fix checkbox styling with dark theme

Signed-off-by: Julius Härtl <jus@bitgrid.net>

apps/accessibility/css/themedark.scss

@@ -74,3 +74,15 @@ $color-border-dark: lighten($color-main-background, 14%);
 		filter: invert(100%);
 	}
 }
+
+input[type=checkbox] {
+	&.checkbox {
+		&:checked + label:before {
+			background-image: url('../../../core/img/actions/checkbox-mark-dark.svg');
+		}
+
+		&:indeterminate + label:before {
+			background-image: url('../../../core/img/actions/checkbox-mixed-dark.svg');
+		}
+	}
+}

core/img/actions/checkbox-mark-dark.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" version="1.1" viewbox="0 0 16 16" width="16" height="16"><path d="M11.924 4.066l-4.932 4.97-2.828-2.83L2.75 7.618l4.242 4.243 6.365-6.365-1.433-1.432z" fill="#000"/></svg>

core/img/actions/checkbox-mixed-dark.svg

@@ -1 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" version="1.1" viewbox="0 0 16 16" width="16" height="16"><path d="M4 7v2h8V7H4z" fill="#fff"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" version="1.1" viewbox="0 0 16 16" width="16" height="16"><path d="M4 7v2h8V7H4z" fill="#000"/></svg>