portico: Define variable for modal background color.

The CSS for modals uses "--color-background-color" to set
background color of modal. But the variable is defined in
zulip.css which is not available for portico pages and thus
the email visibility modal was not rendering as expected in
"Terms of service" page.

This commit defines the "--color-background-color" variable
in portico.css such that it is available for portico pages
as well.

web/styles/portico/portico.css

@@ -1,3 +1,7 @@
+:root {
+    --color-background-modal: hsl(0deg 0% 98%);
+}
+
 body {
     background-color: hsl(0deg 0% 98%);
     font-family: "Source Sans 3", sans-serif;