When I started using WeeChat as my IRC client, I searched around for a light theme to use with it (since I use a light theme for my terminal, and the default WeeChat colors make several UI bits difficult to read with a light terminal theme). I couldn't find much in the way of WeeChat light themes, however, so I set up my own — I went through all the color settings listed in the WeeChat User's Guide, and replaced defaults that showed up as dark-on-dark or light-on-light in my terminal with colors that instead would be dark-on-light or light-on-dark.
These are the settings I ended up modifying to build the light theme I now use (you can look up each setting in the User's Guide for a brief description of each, if you're curious).
/set weechat.bar.status.color_bg gray
/set weechat.bar.title.color_bg gray
/set weechat.color.chat_buffer black
/set weechat.color.chat_channel black
/set weechat.color.chat_nick_offline_highlight_bg gray
/set weechat.color.chat_nick_self darkgray
/set weechat.color.chat_prefix_action darkgray
/set weechat.color.status_data_msg lightblue
/set weechat.color.status_more lightblue
/set weechat.color.status_name darkgray
/set weechat.color.status_number lightblue
/set buflist.format.buffer_current "${color:,yellow}${format_buffer}"
/set buflist.format.hotlist_low "${color:cyan}"
/set irc.color.topic_new red
/set relay.color.text_selected lightblue
There are a lot more color settings, but these were the only ones I needed to change to fix the dark-on-dark and light-on-light issues (I left the other settings alone). I use InWee to manage my custom WeeChat settings; I store these color settings in a file called colors.txt, and then start up WeeChat and run a command like inwee colors.txt whenever I want to make changes.
Here's a screenshot of what it looks like in my termial's color scheme: