Hi,

without addon php code customization is NOT possible. But you can use "Custom" Joomla module put there all HTML code you want, then use it inside Module addon. But first test & publish module in Template Module position if all works.

About button appearance, use bootstrap 5 classes, examples:

<button type="button" class="btn btn-primary">Primary</button>
<button type="button" class="btn btn-secondary">Secondary</button>
<button type="button" class="btn btn-success">Success</button>
<button type="button" class="btn btn-danger">Danger</button>
<button type="button" class="btn btn-warning">Warning</button>
<button type="button" class="btn btn-info">Info</button>
<button type="button" class="btn btn-light">Light</button>
<button type="button" class="btn btn-dark">Dark</button>

 <button onclick="myFunction()" class="btn btn-primary">Click me</button> 

whole rest is in your hands (keyboard) now.