• Use the ES6 export (export default luma) instead of ES5 (module.exports = luma). Let me know if this leads to issues in your installation.
• Use blue color for checkboxes by default
• Add text color modifier classes. You can now use text-primary, text-info, text-success, text-warning and text-danger on any text elements (e.g. p or span) to apply the standard colors to your text.
• I have added a utility class to align items vertically: flex-group.

Example:

<div class="flex-group">
    <h1>Title</h1>
    <button class="is-right">Download</button>
</div>

This would vertically align the h1 and button. Additionally, the button would be aligned to the right side.

• Use different transition for checkboxes and make file selects the same height as other form elements
• Remove padding on container. If this leads to issues, let me know.