Fix: In the dark night theme, the message input box is not displayed correctly. #6950 (#6951)

### What problem does this PR solve?

Fix: In the dark night theme, the message input box is not displayed
correctly. #6950

### Type of change


- [x] New Feature (non-breaking change which adds functionality)

web/src/components/message-input/index.less

@@ -1,7 +1,6 @@
 .messageInputWrapper {
   margin-right: 20px;
   padding: '0px 0px 10px 0px';
-  background-color: #ffffff;
   border: 1px solid #d9d9d9;
   &:hover {
     border-color: #40a9ff;

web/src/components/message-input/index.tsx

@@ -5,6 +5,7 @@ import {
   useRemoveNextDocument,
   useUploadAndParseDocument,
 } from '@/hooks/document-hooks';
+import { cn } from '@/lib/utils';
 import { getExtension } from '@/utils/document-util';
 import { formatBytes } from '@/utils/file-util';
 import {
@@ -222,7 +223,11 @@ const MessageInput = ({
   }, [conversationId, setFileList]);
 
   return (
-    <Flex gap={1} vertical className={styles.messageInputWrapper}>
+    <Flex
+      gap={1}
+      vertical
+      className={cn(styles.messageInputWrapper, 'dark:bg-black')}
+    >
       <TextArea
         size="large"
         placeholder={t('sendPlaceholder')}