To completely overwrite _module.less of Orange_Checkout module in a custom theme, the developer should place the file in the Custom/theme/Orange_Checkout/web/css/source directory. This will override the default _module.less file from the Orange_Checkout module and apply the custom styles to the theme. The Custom/theme/Orange_Checkout/frontend/web/css/_module.less and Custom/theme/web/css/source/Orange_Checkout/_module.less paths are not valid and will not work, as they do not follow the theme structure or the module naming convention. References: [Theme structure], [Module naming convention]