It’s not so much the theme, it’s that WordPress itself doesn’t allow <script>
tags in the content editor. They do this for security reasons.
One option is to build a shortcode, which you can then place in your content.
For example:
add_shortcode( 'my_ad', function() {
?>
<script>YOUR SCRIPT IN HERE</script>
<?php
} );
Then you can add this to your content area: [my_ad]