Sure! it's kinda complicated and i'm not the best at explaining. but i hope you understand :")
- open apk editor and click 'select an apk file'
- choose simple edit
- go to assets > x-games > x-gui > x-phone, you'll see a png file named x-textbox
- save / copy the file to your phone by clicking the floppy disk button
- edit the file using image editor to a darker shade ( or scroll below to use mine instead )
- rename the edited file to x-textbox, this is important!!
- repeat step 1, 2, 3
- replace the old x-textbox by the edited x-textbox using the pen button
- save and install
Enjoy!
( i attached a simple textbox png if you're too lazy to edit one )