Use XML file to order the loading of the CSS files
In Global folder
of your theme file structure, you can place as many CSS files as you
want but if you need to load them in a specific order, you must describe
them in the cssLoadOrder.xml. located in the ~/App_Data/Sitefinity/WebsiteTemplates/YourTemplate/App_Themes/MyTheme/Global/ folder. The file has content similar to the following:
The sequence of loading the files will be according to their arrangement in cssLoadOrder.xml.