Include SELECT2_THEME settings

[codingjoe]

Discussed in #99

^{Originally posted by CleitonDeLima October 29, 2021}
Select2 accepts themes, an example is bootstrap4.

To configure it, you need to include a new css file and add the theme option when starting select2.

<link rel="stylesheet" href="/path/to/select2.css">
<link rel="stylesheet" href="/path/to/select2-bootstrap4.min.css">

$('select').select2({
    theme: 'bootstrap4',
});

The settings could look like this:

# accept str too
SELECT2_CSS = (
    "/path/to/select2.css",
    "/path/to/select2-bootstrap4.min.css",
)
SELECT2_THEME = `"bootstrap4"`

I am willing to develop this option if it is valid.