1) Custom Themes are disabled in new firmwares, they need to be enabled with Custom_Theme_enable.app
2) To be on the save side, copy an actual swupdate.bin into the internal memory of your Pocketbook, in case something goes wrong starting the PB with Left and Right pressed will force the update
3) To get your theme from the device use: save_theme.app, the theme file (probably Line.pbt) will be copied into a folder in the /temp/themes
4) Extract the config.cfg out of the theme you got previously via pbtheme.exe -e Line.pbt config.cfg
5) Edit config.cfg
6) Update config.cfg in the theme via: pbtheme.exe -r Line.pbt config.cfg NewThemeName.pbt
7) Copy NewThemeName.pbt into /system/themes
8) Goto Setting/Personal Settings/Theme  and select NewThemeName.pbt


Credit goes to Lighting with:
https://github.com/Lighting/pbtheme
and
https://github.com/Lighting/Setup-openwith 