Some of new conversations are not using webkit

run pidgin in gdb and give some more info.

did pidgin must be compiled with debug ?

now I just receive
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xb71f5700 (LWP 10404)]
0xb6f54166 in remove_webkit (conv=0x871f3c8, gx=0x871d398, null=0x0) at webkit.c:1323
1323 GTK_PANED(parent)->child1 = NULL;

when uncheck to unload plugin ...

for rest about missing themes in converstions attaching
Screenshot-A3 CbM-drJeckyll.png

What other plugins are you using? Maybe something is interfering with pidgin-webkit since I haven't experienced any non-themed tabs/windows. You can also give output of 'pidgin -d | grep WebKit > debug.log' here as attachment when you get non-themed conversation.

I use webkit-gtk 36403

Already disable all other plugins with no effect

I make 3 debug.log-s
debug1.log - Load pidgin then activate plugin, all open conversations all themed, unload plugin crash
debug2.log - Plugin is already active when loading pidgin - receive auto message from irc which is no themed, and all other converstions after that are not themed
debug3.log - Plugin is already active, no message received, then open new conversation with someone window is correctly themed, when open another is again themed... but when receive answer and open conversion it is not themed ...

my conclusion is that if I start conversation with someone window is correctly themed, but if someone start conversation with me when I open window it is not themed ...

I don't currently know what is wrong in your case, those debug logs look perfectly normal (not counting the crash ;). And what I can guess from that very short backtrace, it seems that it crashes just because your conversation is not showing webkit while the plugin thinks it is.

guess what ... with rev 67, and webkit-gtk p37894 it just works for now :)