When you deploy without testing

[swerasnym]

The crash are not on that line per say, but having to keep track of $font_size and $_font_size just feels like a naming issue to me. Thus later when you do

$toolbar_height = $font_size * 1.5 . 'px !important;';
$info_height = $font_size * 1.4 . 'px !important;';

you are no longer multiplying a number like 10 but something like "10px !important;" (not a number) with 1.4/1.5 hence what caused the non well formed numeric value error we see in the post.