Php developer to fix theme

I have an existing website which has been working fine. I needed to update the PHP version to a more recent one, and that broke the theme. Rolling back to the old version didn’t seem to help so the hosting company put the WordPress default theme on. I need someone who can look at the errors and try to update the theme code. The theme is no longer supported and it was customised also. So I need someone who has experiencing coding and is able to look at the issue and fix it.