register: Add bootstrap CSS rules for select elements.

This commit adds bootstrap CSS rules used for select
elements in realm creation form to the existing CSS
in portico_signin.css.

This change is done so we can safely remove select CSS
rules from bootstrap.css as a part of our process to
remove bootstrap.

static/styles/portico/portico_signin.css

@@ -510,6 +510,9 @@ html {
             border: 1px solid hsl(0, 0%, 87%);
             box-shadow: none;
             border-radius: 4px;
+            background-color: hsl(0, 0%, 100%);
+            color: hsl(0, 0%, 33%);
+            cursor: pointer;
 
             transition: border 0.3s ease;