Banner: improve mobile styles

app/soapbox/components/ui/banner/banner.tsx

@@ -14,11 +14,11 @@ const Banner: React.FC<IBanner> = ({ theme, children, className }) => {
       data-testid='banner'
       className={classNames('fixed bottom-0 left-0 right-0 py-4 z-50', {
         'backdrop-blur bg-primary-900/80': theme === 'frosted',
-        'bg-white dark:bg-slate-800 text-black dark:text-white shadow-lg dark:shadow-inset': theme === 'opaque',
+        'bg-white dark:bg-slate-800 text-black dark:text-white shadow-2xl dark:shadow-inset': theme === 'opaque',
       }, className)}
     >
       <div className='max-w-3xl md:max-w-7xl mx-auto lg:grid lg:grid-cols-12'>
-        <div className='col-span-9 col-start-4 xl:col-start-4 xl:col-span-6 md:px-8'>
+        <div className='col-span-9 col-start-4 xl:col-start-4 xl:col-span-6 px-4 md:px-8'>
           {children}
         </div>
       </div>