add mobile background

conststr.php

@@ -79,6 +79,7 @@ $constStr = [
             'disableChangeTheme' => 'if 1, the Theme selection button will not display',
             'downloadencrypt' => '0 or 1. if 1, the files in encrypt folder can be downloaded without password',
             'background' => 'Set an url as background photo.',
+            'backgroundm' => 'Set an url as background in mobile phone.',
             'theme' => 'Select theme.',
             'timezone' => 'Set default timezone.',
             'guestup_path' => 'Set guest upload dir, before set this, the files in this dir will show as normal.',
@@ -102,6 +103,7 @@ $constStr = [
             'disableChangeTheme' => '如果填 1, 主题选择切换将被隐藏',
             'downloadencrypt' => '0 或 1。如果 1, 那加密目录内的文件可以不需要密码就能下载。',
             'background' => '设置一个url作为背景。',
+            'backgroundm' => '设置一个url作为手机用的背景。',
             'theme' => '选择一个主题。',
             'timezone' => '设置默认时区。',
             'guestup_path' => '设置游客上传路径（图床路径），不设置这个值时该目录内容会正常列文件出来，设置后只有上传界面，不显示其中文件（登录后显示）。',