feat(FE): add custom gray color

Author: Manuel Proß

web/src/styles/globals.scss

@@ -11,6 +11,7 @@
     --color-primary: #3498db;
     --color-secondary: #2ecc71;
     --color-tertiary: #e74c3c;
+    --gray-1: #313131;
     --primary-text: #333;
     --secondary-text: #555;
 }