You're not testing against the value in $currentime but against a str with the value $currentime. You need to remove the quotes around "$currenttime"
Like so:
if [[ $currenttime > "17:00" ]]; then
But it may be waaaay simpler to run the commands in a cronjob. I dont know how you start this script, but if you do it with cronjob and execute every minute then every minute the light program receives a command to adjust the value. I would make more sense to execute the command only on the exact set time you would want to
0 18 * * * light -Srs "sysfs/leds/chromeos::kbd_backlight" 25 >/dev/null 2>&1
0 19 * * * light -Srs "sysfs/leds/chromeos::kbd_backlight" 50 >/dev/null 2>&1
0 20 * * * light -Srs "sysfs/leds/chromeos::kbd_backlight" 100 >/dev/null 2>&1
0 6 * * * light -Srs "sysfs/leds/chromeos::kbd_backlight" 50 >/dev/null 2>&1
0 7 * * * light -Srs "sysfs/leds/chromeos::kbd_backlight" 0 >/dev/null 2>&1
edit:
or run the script every minute and increase the light value by (max_light_val / minutes_till_next_setting). That can be fun :)