Introduce CSS variables for consistent theming

This commit refactors existing styles to use CSS variables, centralizing values for colors, spacing, typography, and other design elements. It also adds a `variables.css` file for managing these variables. Updated affected components and stylesheets for better maintainability and consistency.

Gallus_Pub_v1/styles/components/EventsGrid.css

@@ -2,8 +2,8 @@
   display: flex;
   flex-wrap: wrap;
   justify-content: center;
-  gap: 30px;
-  padding: 2rem 0;
+  gap: var(--gap-standard);
+  padding: var(--padding-vertical) var(--padding-horizontal);
   width: 100%;
 }
 
@@ -17,4 +17,4 @@
   .events-gird {
     justify-content: space-around;
   }
-}
+}