To display the chat window with white text on black background, add a file ~/.purple/gtkrc-2.0 with the following content :
# Create a style called "inverted" where the text and base
# (the base color behind the widget) are the reverse of typical.
style "inverted"
{
text[NORMAL] = "#FFFFFF"
base[NORMAL] = "#000000"
}
# Apply "inverted" to conversation entry box--where you type.
widget "*pidgin_conv_entry" style "inverted"
# Apply "inverted" to conversation history pane--where you read the conversation.
widget "*pidgin_conv_imhtml" style "inverted"
Restart Pidgin and enjoy !
Source : https://developer.pidgin.im/wiki/Using%20Pidgin#HowdoIchangethefontPidginusesThebackgroundcolor