Fix box-shadow value in .wy-side-nav-search custom.css

The box-shadow property in .wy-side-nav-search was incorrectly specified with four values, omitting the necessary units.

Author: cypherpepe

docs/_static/css/custom.css

@@ -229,7 +229,7 @@ small * {
 .wy-side-nav-search {
     background-color: transparent !important;
     color: var(--color-a) !important;
-    box-shadow: 0 4 4 0 var(--color-a);
+    box-shadow: 0 4px 4px 0 var(--color-a);
     border-bottom: 1px solid var(--color-d) !important;
 }
 
@@ -823,4 +823,4 @@ a.skip-to-content:focus {
 #content {
     scroll-margin-top: 6rem;
     scroll-behavior: smooth;
-}
+}