Use `Button` instead of `ButtonBase` in `PaginationNavigation`

src/sidebar/components/PaginationNavigation.tsx

 import {
-  ButtonBase,
+  Button,
   ArrowLeftIcon,
   ArrowRightIcon,
 } from '@hypothesis/frontend-shared';
@@ -16,14 +16,17 @@ type NavigationButtonProps = PresentationalProps &
 
 function NavigationButton({ ...buttonProps }: NavigationButtonProps) {
   return (
-    <ButtonBase
+    <Button
       classes={classnames(
-        'px-3.5 py-2.5 gap-x-1 font-semibold',
+        'px-3.5 py-2.5 gap-x-1',
+        'font-semibold',
         // These colors are the same as the "dark" variant of IconButton
         'text-grey-7 bg-grey-2 enabled:hover:text-grey-9 enabled:hover:bg-grey-3',
         'disabled:text-grey-5 aria-pressed:bg-grey-3 aria-expanded:bg-grey-3'
       )}
       {...buttonProps}
+      size="custom"
+      variant="custom"
     />
   );
 }