style: Fix free disk space caption dot color

It hardcodes a color which matches only in light mode, and only if high
contrast is not enabled.

Instead derive from currentColor to get light/dark automatic support
and double the alpha opacity on high contrast.

The alpha values were copied from libadwaita's color used by level bar.

src/resources/style-hc.css

@@ -10,3 +10,7 @@
 #NautilusQueryEditorTag {
   box-shadow: inset 0 0 0 1px @borders;
 }
+
+.disk-space-free {
+  color: alpha(currentColor, 0.3);
+}

src/resources/style.css

@@ -118,7 +118,7 @@
 }
 
 .disk-space-free {
-  color: alpha(@dark_5, 0.12);
+  color: alpha(currentColor, 0.15);
 }
 .disk-space-used {
   color: @accent_bg_color;