Be aware that LESS files need to be processed into CSS files to work. If your developer tools are showing styles as coming from a .less file then your theme likely has a source map. This is typically only for development purposes only. The actual styles will be coming from a .css file somewhere. You should ask your theme author for specifics.
That being said you should edit CSS files in your theme. Any changes you make will be erased when/if the theme is updated. Instead add your custom CSS to the Additional CSS section of the Customizer.
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