it looks like the typo is in the beginning of the install/install_styles.php script
FIND (at the top):
if ($style_is_active) {
$style_is_active = 1;
}
REPLACE WITH:
$style_is_active = ($style_is_active) ? 1 : 0;
The bug presents itself when you've selected to have the upgrader upgrade your currently installed styles for you, and you have styles that are marked as not active. The null variable would pass in PHP 5.x (as "0"), but in PHP 7+ it is seen as missing.
As this code does not exist in any prior versions, this only effects version 7.7.3.
The install/install_styles.php script feature has since been rewritten for 7.7.4.
