Select

Use .select class to set quick and default select style.

Size

.select for default size select field
.select-xs for small size select field
.select-md for medium size select field
.select-lg for large size select field
<select class="select-xs">...</select>
<select class="select">...</select>
<select class="select-md">...</select>
<select class="select-lg">...</select>
    
<select class="select" multiple size="3">...</select>

Width

Use .full or .half class for 100% or 50% width. 
<select class="select half" multiple size="3">...</select>
<select class="select full">...</select>

Disabled

Just add disabled attribute to make field disable

<select class="select" multiple size="3" disabled>...</select>
<select class="select" disabled>...</select>

Round

To make border round use border radius shorthand .br-round

<select class="select br-round">...</select>

Shadow

To add shadow use shadow shorthand like .shadow or .shadow-1

<select class="select shadow-1">...</select>

Color

To add the color add shorthand classes for border color, text color, placeholder color and focus color.

<select class="select focus-indigo border-indigo bg-indigo white">...</select>