Fit the tooltip shadow to the default variable

Signed-off-by: John Molakvoæ (skjnldsv) <skjnldsv@protonmail.com>

core/css/tooltip.scss

@@ -115,7 +115,7 @@
 	padding: 5px 8px;
 	background-color: $color-main-background;
 	color: $color-main-text;
-	box-shadow: 0 1px 10px rgba(150, 150, 150, .75);
+	box-shadow: 0 1px 10px $color-box-shadow;
 	text-align: center;
 	border-radius: 3px;
 }