Fix for 27eb9e - "Changing color of logo from black to gray"

Use a variable consistent with Wikimedia UI base instead of
a hard-coded opacity value.

Bug: T251135
Change-Id: I883d3b122c3a0206fe83365a7818929d74f86459

minerva.less/minerva.variables.less

@@ -136,6 +136,7 @@
 @notificationColorUnread: #fff;
 
 // Opacity
+@opacity-icon-base: 0.87; // = `#222` on `background-color: #fff`, closest to `#202122`
 @opacity-icon-subtle: 0.66; // = `#565656 on `background-color: #fff`, closest to `#54595d`
 
 // z-index:

resources/skins.minerva.base.styles/ui.less

@@ -93,6 +93,7 @@ main {
 
 	.branding-box {
 		width: auto;
+		opacity: @opacity-icon-base; // T251135
 
 		// TODO: Remove `h1` after T198947 HTML changes expire from cache.
 		h1,