Sidney Pro II Header

It appears someone hacked into my site. I now get some text that pops onto the screen before my site comes up.
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Notice: Use of undefined constant HTTP_USER_AGENT - assumed ‘HTTP_USER_AGENT’ in /home2/rickfromm/apexdataconsulting.com/wp-content/themes/sydney-pro-ii/header.php on line 1

If you goto apexdataconsulting.com you will see this happen.
Also, every now and then it gets completely redirected.

Is there a way to reinstall the theme without losing any data?

Data will stay there after re-install. Visit your site’s folder via cPanel of FTP program. Got to wp-content folder and remove the sydney-pro-ii folder. And then re-upload the theme.

If you feel unsure for loosing data, make a backup first with a plugin like https://wordpress.org/plugins/backupwordpress/.

Regards,
Kharis