Hi,
By default, the folder is not included so please add new folder and .less file and go ahead to next steps.
In case you want to load style for a specific layout or page, you can use the instruction below.
Way 1:
1. Create style file (.less file) in the less folder that you want to load to the menu items you want (for example: blog.less)
2. Open the layout file that the menu items you want to load the css file and add the following code:
<head>
<?php $this->addCss('layouts/blog') ?>
</head>
Way 2:
1. Create style file (.less file) in the folder: templates\t3_bs3_blank\less\extras
2. Open any t3 bs3 blank template style, in the "Add-ons" tab, you will the section: Template Extended styles, you can assign the pages that will load the custom style from the .less file you created in step 1.
Regards.