Make alt text readable when the logo doesn't load

We were displaying the alt text using the same color as the background
color, which made it impossible to read it when the logo didn't load
(for whatever reason).

Using the same color as the text, like done in the admin section, solves
the issue.

app/assets/stylesheets/layout/admin_header.scss

@@ -54,7 +54,6 @@
       }
 
       a {
-        color: inherit;
         display: inline-block;
         font-family: "Lato" !important;
         font-size: rem-calc(24);