I'm trying to decrease the white border thickness in forum/board listing to something like 1px, but I can't seem to find it anywhere in the default CSS file.. could anyone give me some directions?
I know Ich Bin has already given you a correct answer for this, but if you are interested in taking the concept further there is another option that can be implemented just by CSS changes.
What can you do is use border-collapse: collapse; on the table_list class in index.css. This will remove all visible borders between table cells and rows. You can also do this by using cellspacing="0" in the template, but border-collapse has an advantage.
What it allows you to do is choose which cells have borders and where they have them. It removes the default gaps, but still allows setting a border via CSS. If you only want borders between rows, you can have those without any others. If you want to remove the vertical border between the on/off icon and the board name, you can do that. Etc, etc.
If you are interested in playing around with this, I suggest checking out the CSS for the table_grid class.
@ NBK*Twitch: no that's not correct.
To change that stuff, looks for #forumposts in index.css. Specifically, you want top and bottom margins for div.cat_bar and div.windowbg and div.windowbg2.