The theme offers styles to display "small" and "large" Select2 widgets in Bootstrap Input Groups with Bootstrap Control Sizing classes applied (e. g. Select2 in .input-group.input-group-sm or .input-group.input-group-lg). You may also apply the Bootstrap Control Sizing classes directly to the .select2-container to alter its appearance.
Those of you familiar with Select2 v3 remember that the default behavior was to copy the original elements CSS-classes to the container of the Select2 element. Select2 v4 provides a similar functionality in its full build, select2.full.js. It contains a compatibility module which behaves similar to v3 in copying CSS classes from the original <select> element. To invoke, set the containerCssClass option to :all:.
The theme's styles work in Bootstrap's Horizontal Forms and offers support for Horizontal Form Group Sizes, too.
Tests for Select2 widgets used in context with Bootstrap's Button Addons.
The theme applies Bootstrap's styles for disabled input elements and for disabled dropdown options to the Select2 widgets and its options. Also see Select2's documentation on its "Disabled mode".