Open themes/rainy_day/theme.php, and right near the top is the code for the main menu. Find;
// HTML template for main menu
In rainy day, the main menu is split into 2 sections;
$template_main_menu1
and;
$template_main_menu2
First, DO NOT remove any of the existing stuff in comment marks, '<!-- Stuff like this -->'.
You will see that each button consists of 4 cells, such as this;
<!-- BEGIN my_gallery -->
<td><img name="spacer" src="images/spacer.gif" width="5" height="25" border="0" id="spacer" alt="" /></td>
<td><img name="button1_r1_c1" src="themes/rainy_day/images/button1_r1_c1.gif" width="5" height="25" border="0" id="button1_r1_c1" alt="" /></td>
<td background="themes/rainy_day/images/button1_r1_c2.gif">
<a href="{MY_GAL_TGT}" title="{MY_GAL_TITLE}">{MY_GAL_LNK}</a>
</td>
<td><img name="button1_r1_c3" src="themes/rainy_day/images/button1_r1_c3.gif" width="5" height="25" border="0" id="button1_r1_c3" alt="" /></td>
<!-- END my_gallery -->
You should comment out all the buttons you don't want, so the above code would become this;
<!-- BEGIN my_gallery --><!--
<td><img name="spacer" src="images/spacer.gif" width="5" height="25" border="0" id="spacer" alt="" /></td>
<td><img name="button1_r1_c1" src="themes/rainy_day/images/button1_r1_c1.gif" width="5" height="25" border="0" id="button1_r1_c1" alt="" /></td>
<td background="themes/rainy_day/images/button1_r1_c2.gif">
<a href="{MY_GAL_TGT}" title="{MY_GAL_TITLE}">{MY_GAL_LNK}</a>
</td>
<td><img name="button1_r1_c3" src="themes/rainy_day/images/button1_r1_c3.gif" width="5" height="25" border="0" id="button1_r1_c3" alt="" /></td> -->
<!-- END my_gallery -->
Once you have done that, move the ones you do want, from template main menu2 into template main menu1. If they have the comments that start and end the section, those comments must go with them.