The pixels for function keys are stored in
/u/ccr.17/pixmap
You need to know the screen resolution first easiest way, run sieat, at prompt, when you exit it will say
2048x1152.qss
change directory to this resolution
/u/ccr.17/pixmap/2048x1152.qss
under these directories there is
tk_f1_on.png dk_f1_on.png ek_f1_on.png
tk - transaction key dk - default key ek - extension key (row 2 sieat - eg. alter orders)
F1 - function keys - there will be one for each function key
you need to edit these files so you can look at what pixel they have
gthumb tk_f1_on.png should say something like 130x95
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to see the resolution you can also do this:
export SHOW_GUI_DETAILS=1
then run sieat and mouse over the function key this will show /u/ccr.17/pixmap/2048x1152.qss