In my project, there is Megamenu implemented and to customization of Megamenu, I have used
custom _module.less in my theme folder By overriding the Megamenu Folder.
app->code->RB->Megamenu (is the Megamenu that i have implemented).which contains its core files (Megamenu module folder)
app->design->mytheme->RB_Megamenu->web>source->css->_module.less (In this file i have put customizing Megamenu css)
Now the issue is whenever I disable Megamenu and the default Magento Menu is Enabled, The default Magento menu taking CSS of from Megamenu which creates confliction between two CSS file.