First of all, 3aaamel eh ya MASRYYYY ??
Now that that’s out of the way, thematic_header() is obviously a function, so clearly it is calling it from somewhere. I’m still a beginner to WP, but I do know code. Look through the theme files in the editor to try and find where thematic_header() is defined. If it’s not then it must be pre-defined in the WP API.
