[Glitch] Add color variables of texts for better accesibility

Port 74dae9458d and related to glitch-soc

app/javascript/flavours/glitch/styles/basics.scss

@@ -71,7 +71,7 @@ body {
   &.error {
     position: absolute;
     text-align: center;
-    color: $ui-primary-color;
+    color: $darker-text-color;
     background: $ui-base-color;
     width: 100%;
     height: 100%;