css: Fix modal exit button colors not working on billing pages.

This was due to color variables not being accessible as `zulip.css`
is not a file we import on billing page.

web/styles/modal.css

@@ -1,4 +1,13 @@
 /* Styles for the Micromodal-based modals */
+:root {
+    /* Exit buttons are sometimes Cancel, but sometimes
+       other "Nah, forget it" actions. */
+    --color-exit-button-text: hsl(0deg 0% 0%);
+    --color-exit-button-border: hsl(300deg 2% 11% / 30%);
+    --color-exit-button-background: hsl(0deg 0% 100%);
+    --color-exit-button-background-interactive: hsl(0deg 0% 97%);
+}
+
 .modal__overlay {
     position: fixed;
     inset: 0;

web/styles/zulip.css

@@ -418,12 +418,9 @@ body {
     --icon-chevron-down: url("data:image/svg+xml,%3Csvg width='16' height='16' viewBox='0 0 16 16' fill='none' xmlns='http://www.w3.org/2000/svg'%3E%3Cpath fill-rule='evenodd' clip-rule='evenodd' d='M3.52977 5.52973C3.78947 5.27004 4.21053 5.27004 4.47023 5.52973L8 9.05951L11.5298 5.52973C11.7895 5.27004 12.2105 5.27004 12.4702 5.52973C12.7299 5.78943 12.7299 6.21049 12.4702 6.47019L8.47023 10.4702C8.21053 10.7299 7.78947 10.7299 7.52977 10.4702L3.52977 6.47019C3.27008 6.21049 3.27008 5.78943 3.52977 5.52973Z' fill='%23333333'/%3E%3C/svg%3E%0A");
 
     /* Button colors on modals and message editing. */
-    /* Exit buttons are sometimes Cancel, but sometimes
+    /* Don't define light theme colors for modal here
-       other "Nah, forget it" actions. */
+       since the modal is also used on billing pages and
-    --color-exit-button-text: hsl(0deg 0% 0%);
+       this file is not imported on billing pages. */
-    --color-exit-button-border: hsl(300deg 2% 11% / 30%);
-    --color-exit-button-background: hsl(0deg 0% 100%);
-    --color-exit-button-background-interactive: hsl(0deg 0% 97%);
 }
 
 %dark-theme {