fix: replace border-gray-300 with a themable colour (#664)

Co-authored-by: Aman Harwara <amanharwara@protonmail.com>

app/assets/javascripts/preferences/components/Content.tsx

@@ -18,8 +18,8 @@ export const Text: FunctionComponent<{ className?: string }> = ({
 }) => <p className={`${className} text-xs`}>{children}</p>;
 
 const buttonClasses = `block bg-default color-text rounded border-solid \
-border-1 border-gray-300 px-4 py-1.75 font-bold text-sm fit-content \
-focus:bg-contrast hover:bg-contrast`;
+border-1 px-4 py-1.75 font-bold text-sm fit-content \
+focus:bg-contrast hover:bg-contrast border-neutral`;
 
 export const LinkButton: FunctionComponent<{
   label: string;