Magento 2 : How to Import Custom Module’s Less file in Custom Theme

I am creating custom theme without parent. Now, I want to add custom less file inside Magento_Theme folder.

File path : app/design/frontend/VendorName/ThemeName/Magento_Theme/web/css/source/_mycustom.less

I tried to call this file inside app/design/frontend/VendorName/ThemeName/web/css/_sources.less file. But, it’s not working. I added this below line in _sources.less file

@import '../Magento_Theme/web/css/source/_mycustom.less';

How can I call it without _module.less file?

Anyhelp would be appreciated.