Hello,
MDL-69491 has just landed in Moodle 3.8 - 4.0 which fixes the 'A non well formed numeric value encountered' error that can appear in the PHP log when using themes like Adaptable and setting a setting that causes the CSS to be recreated.
Thus to get this fix, please update your core Moodle with 'Build: 20200903' or above.
Gareth