Make sure you've got Less (http://lesscss.org/) up and running before starting with Strapless. Basic understanding of Less scope, Less importing and, well, just Less in general is recommended.
Use Strapless as the base of your project by renaming strapless.less into [your project name here].less. Or index.less. Or whatever you want. It's up to you. Strapless is intended as a starting point for your project's CSS/Less.
To add your own CSS rules, simply insert your own Less after the core Strapless library files. You can also edit the Strapless library directly, just remember anything included as a (reference) in strapless.less doesn't output CSS directly, and editing library files might make it harder to update Strapless in the future. If you find yourself overwriting too many rules for a particular element, you can start from scratch by commenting it out in strapless.less.
Strapless is really opinionated in its approach. It's okay if you comment a few things out. We won't be offended.
Here's what's included:
settings.less
Project settings are defined here. Edit them here or redefine them later and let lazy loading do its thing.
mixins.less
Strapless has a few mixins for determining text contrast. They're all determined to do the same thing: Make text and background colors work together to achieve target contrast ratios.
colors.less
Generates all of Strapless's color variables. These are documented in settings.less in variable name lists. (Strapless uses variable name lists because variable names sometimes end up as CSS classes.)
patterns.less
Contains mixins for generating patterns.
targets.less
Ruleset mixins for different targets, including responsiveness targets.
elements/
The elements folder contains Less for HTML elements. Each file is named for its element, while etc.less is a catch-all for rules too small for their own file.
utilities.less
CSS class utilities.