First, you need to find the gtkrc file.

In windows, it's usually at: C:\Program Files\Pidgin\Gtk\etc\gtk-2.0

In linux, it's usually at: ~/.purple$ cat gtkrc-2.0 

# Create a style called "satanic" where the text and base colors can be set:
style "satanic"
{
   base[NORMAL] = "#CCCCCC"
   text[NORMAL] = "#000000"
}

# Apply "satanic" to conversation entry box--where you type.
widget "*pidgin_conv_entry" style "satanic"

# Apply "satanic" to conversation history pane--where you read the conversation.
widget "*pidgin_conv_imhtml" style "satanic"