As far as i know, their might be 3 cause
1) Greenfoot was running on background. If yes, then close it and open it again
2) U haven't replaced it. U might have clicked the cancel button. Try removing original css file temporarily and then paste the css flie which I edited
3) U haven't placed it in correct folder.
2021/5/17
Bouncing Bullets Tank AI Demo
2021/5/16
DARK THEME
2021/5/15
DARK THEME
2021/5/12
DARK THEME
2021/5/12
DARK THEME
2021/5/11
DARK THEME
2021/5/11
DARK THEME
2021/5/11
DARK THEME
2021/5/11
DARK THEME