Fran, here's another style to try. I did type up some explanatory stuff but then I lost it testing another idea and I don't have time to write it all again. I think you'll like this one better.

When you type in new hex numbers for colors, make sure there is no gap between the '#' symbol and the number. It should be (for example):
#6666FF
not

6666FF

I think this may be why parts of your styles revert to the Rendo default colours.

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BODY {background-color:#791fac; color: #000000; }
.content_cell { background-color:#6666FF; }

table.outside_table { width: auto; !important;}

a {font-family: Verdana, Arial, Helvetica, sans-serif; text-decoration: underline; font-size: 12px; color: #000000;}
a:link {color: #000000;}
a:hover {color: #EED1FF;}
a:visited {color: #BF0000;}
select {z-index: -1;}

/* temporary fix for forum_message_cell bug */
tr[bgcolor] {background-color: #6666FF; color: #000000;}

/* Sets the appearance of entry boxes */
textarea {background-color:#7777FF; color: #000000;}

iframe { width: 720px !important;}

/* Sets the appearance of select menus */
select { background-color:#7777FF; color: #000000;}
 
td.content_cell {  background-color: #6666FF; color: #000000;}
.forum_headline {font-size:18px; font-weight:bold; padding-top:4px;}

#forum_announcement { background-color: #6666FF; border:3px solid #791fac; font-color: #000000;}

#forum_announcement td {text-align:center; padding:2px;}

#forum_outer_table {width: 100%; border:2px solid #000000;}
#forum_outer_table td {padding:1px;}

#forum_inner_table { background-color: #000000; color: #000000;}
#forum_inner_table th {background-color:#7D2FAA; text-align:center; font-weight:bold; color:#000000; font-size:11px; height:20px;}
#forum_inner_table td { padding-top:2px; padding-bottom:4px;}

.forum_marker_cell { background-color:#7D2FAA; }
td.forum_name_cell { background-color:#791fac; padding-bottom:5px; }
a.forum_forumname_link, a.forum_forumname_link:hover, a.forum_forumname_link:visited, a.forum_forumname_link:active { font-size:13px; }

a.forum_threadname_link, a.forum_threadname_link:hover, a.forum_threadname_link:visited, a.forum_threadname_link:active { font-size:12px; }
.forum_summary_div { font-size:11px; }
.forum_author_cell { background-color:#7D2FAA; }
a.forum_author_link { color: #000000; }
a.forum_author_link:hover, a.forum_author_link:active, a.forum_author_link:visited { font-size:11px; }
.forum_threads_cell { background-color:#7D2FAA; #color: #000000; font-size:11px; }
.forum_posts_cell { background-color:#791fac; color: #000000; font-size:11px; }

/* sets appearance for number of views column */
.forum_views_cell { background-color:#791fac; font-size:11px; }

/* sets appearance of last posted column */
.forum_last_post_cell { background-color:#7D2FAA; font-size:10px; }

/* sets appearance of group listing */
.forum_group_row_cell { background-color:#6666FF; font-size:14px; font-weight:bold; }

/* The following classes set avatar area appearance */
.forum_avatar_cell { width: 8%; background-color:#6666FF; font-size:11px; padding:2px; padding-top:5px; color:#000000; }
a.forum_avatar_link, a.forum_avatar_link:hover, a.forum_avatar_link:active, a.forum_avatar_link:visited { font-size:10px; };

/* sets appearance of forum entrys */
.forum_message_cell { background-color:#6666FF; color:#000000; }

/* sets descriptive caption for forum entrys */
.forum_message_detail_table { border-bottom:1px solid #791fac; padding-top:2px; padding-bottom:2px; }
.forum_message_detail_cell { background-color: #6666FF; color: #000000; font-size:10px; font-weight: bold; }
.forum_divider_cell { background-color:#791fac; }

/* sets the appearance of quoting in forums */
.forum_quote_div { font-style:italic; padding:12px; color: #000000; background-color: #7777FF; width: 600px; -moz-border-radius: 13px; }
*/

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