fixed slate grey per feedback

=> the darker colour in the style guide is here, it's just called navy-grey; off-black is something else

hot_osm/static/css/hot_osm.css

@@ -8,7 +8,7 @@
     --hot-navy: #20365B;
     --hot-navy-grey: #2C3038;
     --hot-dark-grey: #68707F;
-    --hot-slate-grey: #929D83;
+    --hot-slate-grey: #929DB3;
     --hot-light-grey: #E1E0E0;
     --hot-off-white: #F0EFEF;
     --hot-barely-not-white: #FBFBFB;