Shoutbox

H'mm...I guess it's with the style 923.
Open it and search for

IMInput[id=atom(idSearchText)]

You should find this:

"element[id=atom(idmetoolbarcontainer)]
{
foreground:rcclr(639);
background:rcbkd(636);
}
IMInput[id=atom(idSearchText)]
{
foreground:rgb(51,51,51);
richeditstyle:PlainText|SaveSelection|AllowBeep;   
contentalign:middleleft;.....

Now add background:argb(0,0,0,0); to it, like this:

IMInput[id=atom(idSearchText)]
{
foreground:rgb(51,51,51);
richeditstyle:PlainText|SaveSelection|AllowBeep;   
contentalign:middleleft;
background:argb(0,0,0,0);
(...and more codes down here...)
Down there, there will be something like this(mine is already changed)

element[id=atom(idSearchTextBackground)]
{
contentalign:middleleft;
Foreground:argb(0,172,168,153);
background:argb(0,0,0,0);
}

I don't know if the foreground and background will be there, if they are, put background to argb:(0,0,0,0); and foreground to a colour of your choice, if there aren't just add them.