If you use your web-browser's built-in Developer Tools to Inspect the HTML used to display the Save and Delete buttons on the Saves Dialog you will see it looks something like the following.
<tr>
<td>1</td>
<td>
<button id="saves-save-0" class="save ui-close" type="button" tabindex="0" aria-label="Save Slot 1" title="Save Slot 1">Save</button>
</td>
<td class="empty">
<em>— slot empty —</em>
</td>
<td>
<button id="saves-delete-0" class="delete" disabled="">Delete</button>
</td>
</tr>
If you select each of the two button elements in the Elements tab (or whatever your web-browser calls it) you will see that their associated colour related CSS looks something like the following.
button {
color: #eee;
background-color: #35a;
border: 1px solid #57c;
}
button:disabled {
background-color: #444;
border: 1px solid #666;
}
Adding similar CSS to your story's stylesheet tagged Passage will allow you to modify the colours.
NOTE: The button element CSS selectors also effects buttons not in the Save Dialog, if you want to only target those buttons then alter the selectors like so:
#saves-list button {
color: #eee;
background-color: #35a;
border: 1px solid #57c;
}
#saves-list button:disabled {
background-color: #444;
border: 1px solid #666;
}