Put an empty file in theme_dir/web/css/source/_theme.less. The blank file will override the _theme.less file. After that run :

php bin/magento cache:clean , 
php bin/magento cache:flush and 
php bin/magento setup:static-content:deploy

Put an empty file in theme_dir/web/css/source/_theme.less. The blank file will override the _theme.less file. After that run :

php bin/magento cache:clean
php bin/magento cache:flush 
php bin/magento setup:static-content:deploy

Put an empty file in theme_dir/web/css/source/_theme.less. The blank file will override the _theme.less file. After that run : php bin/magento cache:clean , php bin/magento cache:flush and php bin/magento setup:static-content:deploy

Put an empty file in theme_dir/web/css/source/_theme.less. The blank file will override the _theme.less file. After that run :

php bin/magento cache:clean , 
php bin/magento cache:flush and 
php bin/magento setup:static-content:deploy