Add configurable panel opacity

Closes #68.

src/rulesets/base.less

@@ -63,7 +63,11 @@
 		height: 100%;
 
 		// Set the default font family
-		.use-font-group(text)
+		.use-font-group(text);
+
+		// Set the opacity of the glass panels
+		--b-panel-initial-opacity: 1.0;
+		--b-panel-nested-opacity: 1.0;
 	}
 
 	//</editor-fold>
@@ -209,7 +213,7 @@
 		gap: 8px;
 
 		// Give the first panel in a stack a fixed background color, so it may be seen against a background
-		background-color: .hsl-group(background)[@c];
+		background-color: .hsl-group(background; var(--b-panel-initial-opacity))[@c];
 
 		// Set a min-width for the first level panel
 		@media (max-width: 492px) {
@@ -248,6 +252,8 @@
 		// Do not set min-width for nested panels
 		@{panel} {
 			min-width: unset;
+
+			background-color: .hsl-group(background; var(--b-panel-nested-opacity))[@c];
 		}
 	}
 

src/rulesets/classic.less

@@ -11,6 +11,9 @@
 
 		// Set the shadow drawn on element placed directly on the background
 		--b-outer-shadow: 2px 2px 4px .hsl-group(background)[@c];
+
+		// Override the nested panel opacity to keep them readable
+		--b-panel-nested-opacity: 0.075;
 	}
 
 	&, @{all} {
@@ -38,7 +41,7 @@
 
 		// Do not give the same effects to nested panels
 		@{panel} {
-			background-color: .hsl-group(current; 0.075)[@c];
+			background-color: .hsl-group(current; var(--b-panel-nested-opacity))[@c];
 			backdrop-filter: none;
 			box-shadow: none;
 		}

src/rulesets/glass.less

@@ -8,7 +8,7 @@
 		// Sets the backdrop-filter blur radius of all panels
 		--b-panel-blur: 16px;
 
-		// Set the opacity of the glass panels
+		// Override the opacity of the glass panels
 		--b-panel-initial-opacity: 0.300;
 		--b-panel-nested-opacity: 0.075;
 	}