Simplest way to extend parent styles
To extend the parent theme’s styles in your theme:
In your theme directory, create a web/css/source sub-directory.
Create a _extend.less file there. The path to it looks like following:
<theme_dir>/
│ ├── web/
│ │ ├── css/
│ │ │ ├── source/
│ │ │ ├──_extend.less
...
Add your Less code in this file.
Extending a theme using _extend.less is the simplest option when you are happy with everything the parent theme has, but want to add more styles.
See: https://devdocs.magento.com/guides/v2.4/frontend-dev-guide/css-guide/css_quick_guide_approach.html#simple_extend
To use your custom file create the in your theme web/css/source/_extend.less
and:
@import 'themeDir\web\css\source_theme.less';