The Built-in Stylesheets section of the SugarCube documentation includes a link to ui-dialog's default CSS.
If you open the SAVES dialog within your story you can use your web-browser's built-in Web Developer Tools to Inspect the HTML content of that dialog. It will look something like the following.
<div id="ui-dialog" role="dialog" aria-labelledby="ui-dialog-title" class="open" style="left: 607px; right: 607px; top: 16px; bottom: 16px;">
<div id="ui-dialog-titlebar">
<h1 id="ui-dialog-title">Saves</h1>
<button id="ui-dialog-close" class="ui-close" aria-label="Close"></button>
</div>
<div id="ui-dialog-body" class="saves">
<table id="saves-list">
<tbody>
<tr>
<td>1</td>
<td><button id="saves-save-0" class="save ui-close" type="button" aria-label="Save Slot 1" title="Save Slot 1">Save</button></td>
<td class="empty">• • •</td>
<td><button id="saves-delete-0" class="delete" disabled="" aria-disabled="">Delete</button></td>
</tr>
...Other Slots...
</tbody>
</table>
<ul class="buttons">
<li><button id="saves-export" class="ui-close" type="button">Save to Disk…</button></li>
<li><button id="saves-import" type="button">Load from Disk…</button></li>
<li><button id="saves-clear" disabled="" aria-disabled="">Delete All</button></li>
</ul>
...hidden element...
</div>
</div>
...and you will be able to determine which element IDs and CSS classes are used to effect the different parts that make up that dialog's HTML structure. You can use this information to create CSS rules within your project's Story Stylesheet area.
eg. If you wanted to change the background colour of all the 'Save' buttons (CSS class .save) in the saves list (element ID #saves-list) to green...
#saves-list .save {
background-color: green;
}