Well, you specified your album list to be 5 columns wide. If you only have four albums in a category, the fifth column will remain empty. What magic should coppermine apply to fill that space? The background of the table cell is hardly what I'd call a button, but if you want to change the class of that table cell, edit themes/yourtheme/theme.php, find the definition for
$template_album_list and edit as suggested below. If your custom theme doesn't contain a definition for that variable, then copy
// HTML template for the album list
$template_album_list = <<<EOT
<!-- BEGIN stat_row -->
<tr>
<td colspan="{COLUMNS}" class="tableh1" align="center"><span class="statlink">{STATISTICS}</span></td>
</tr>
<!-- END stat_row -->
<!-- BEGIN header -->
<tr class="tableb_compact">
<!-- END header -->
<!-- BEGIN album_cell -->
<td width="{COL_WIDTH}%" valign="top">
<table width="100%" cellspacing="0" cellpadding="0">
<tr>
<td colspan="3" height="1" align="left" valign="top" class="tableh2">
<span class="alblink"><a href="{ALB_LINK_TGT}"><b>{ALBUM_TITLE}</b></a></span>
</td>
</tr>
<tr>
<td colspan="3">
<img src="images/spacer.gif" width="1" height="1" border="0" alt="" /><br />
</td>
</tr>
<tr>
<td align="center" valign="middle" class="thumbnails">
<img src="images/spacer.gif" width="{THUMB_CELL_WIDTH}" height="1" class="image" style="margin-top: 0px; margin-bottom: 0px; border: none;" alt="" /><br />
<a href="{ALB_LINK_TGT}" class="albums">{ALB_LINK_PIC}<br /></a>
</td>
<td>
<img src="images/spacer.gif" width="1" height="1" border="0" alt="" />
</td>
<td width="100%" valign="top" align="left" class="tableb_compact">
{ADMIN_MENU}
<p>{ALB_DESC}</p>
<p class="album_stat">{ALB_INFOS}</p>
</td>
</tr>
</table>
</td>
<!-- END album_cell -->
<!-- BEGIN empty_cell -->
<td width="{COL_WIDTH}%" valign="top">
<table width="100%" cellspacing="0" cellpadding="0">
<tr>
<td height="1" valign="top" class="tableh2">
<b> </b>
</td>
</tr>
<tr>
<td>
<img src="images/spacer.gif" width="1" height="1" border="0" alt="" /><br />
</td>
</tr>
<tr>
<td width="100%" valign="top" class="tableb_compact">
<div class="thumbnails" style="background-color:transparent"><img src="images/spacer.gif" width="1" height="1" border="0" class="image" style="border:0;margin-top:1px;margin-bottom:0" alt="" /></div>
</td>
</tr>
</table>
</td>
<!-- END empty_cell -->
<!-- BEGIN row_separator -->
</tr>
<tr class="tableb_compact">
<!-- END row_separator -->
<!-- BEGIN footer -->
</tr>
<!-- END footer -->
<!-- BEGIN tabs -->
<tr>
<td colspan="{COLUMNS}" style="padding: 0px;">
<table width="100%" cellspacing="0" cellpadding="0">
<tr>
{TABS}
</tr>
</table>
</td>
</tr>
<!-- END tabs -->
<!-- BEGIN spacer -->
<img src="images/spacer.gif" width="1" height="7" border="" alt="" /><br />
<!-- END spacer -->
EOT;
from themes/sample/theme.php into a new line before
?>
of the file themes/yourtheme/theme.php
To ochange the background color of your empty cells, find
<!-- BEGIN empty_cell -->
<td width="{COL_WIDTH}%" valign="top">
<table width="100%" cellspacing="0" cellpadding="0">
<tr>
<td height="1" valign="top" class="tableh2">
<b> </b>
</td>
</tr>
<tr>
<td>
<img src="images/spacer.gif" width="1" height="1" border="0" alt="" /><br />
</td>
</tr>
<tr>
<td width="100%" valign="top" class="tableb_compact">
<div class="thumbnails" style="background-color:transparent"><img src="images/spacer.gif" width="1" height="1" border="0" class="image" style="border:0;margin-top:1px;margin-bottom:0" alt="" /></div>
</td>
</tr>
</table>
</td>
<!-- END empty_cell -->
in the code you just pasted in and change that to
<!-- BEGIN empty_cell -->
<td width="{COL_WIDTH}%" valign="top">
<table width="100%" cellspacing="0" cellpadding="0">
<tr>
<td height="1" valign="top" class="tableb_compact">
<b> </b>
</td>
</tr>
<tr>
<td>
<img src="images/spacer.gif" width="1" height="1" border="0" alt="" /><br />
</td>
</tr>
<tr>
<td width="100%" valign="top" class="tableb_compact">
<div class="thumbnails" style="background-color:transparent"><img src="images/spacer.gif" width="1" height="1" border="0" class="image" style="border:0;margin-top:1px;margin-bottom:0" alt="" /></div>
</td>
</tr>
</table>
</td>
<!-- END empty_cell -->
Before you do all that, remember not to use the default theme names for custom themes, so do as suggested in
http://coppermine-gallery.net/demo/cpg14x/docs/index.htm#theme_create_rename first before doing anything else.