Countdown Widget has the ability to suit your theme’s stylesheet and it can be customized easily using the Custom CSS field in WordPress Customizer. If you need to add semi-colons between digits of your countdown widget, you can easily do this using the code snippet below:

/* ---------------- Custom CSS ---------------- */
/* Add semi-colons in between countdown digits */
.countdown_section{ position: relative; }
.countdown_amount{ display: inline-block; width: 60px }
.countdown_amount:after{ display: block; position: absolute; content: " : "; top: -3px; left: 97% }
.countdown_row .countdown_section:last-child .countdown_amount:after{ content: "" }

Add this code to the custom css field in your wordpress customizer and it will magically add semicolons to your countdown widget.

You can also use this codepen to design/customize your countdown widget. It includes all HTML and CSS codes included with the latest version of the Countdown Widget plugin.