Finally, I can change Line theme even I didn't jailbreak, by using iTools.
I'm not sure which category should I submit.
I think you can find the step through google. I didn't post the link because I don't know how can I search the tutorial in English.
Anyway, I've tried to explain the steps :
Sync your mobile phone with computer, and open LINE as multitasking. Use "iTools"
> applications [of your phone] > browse at LINE application > Library > Application Support > Theme Packages > a0768339-c2d3-4189-9653-2909e9bb6f58 > images
Edit those images in that "images' folder, and enjoy your custom theme.
[Don't forget to save the picture with transparent bg.]