How to make silver backgroud class code correct

If you are passing this task: create a silver-backgroud class and make backgroud color of div become silver,

and you find you have written a correct code,but it did’t work,

you need to check your input method:

or you just use cntrl+c and cntrl+v rather than use your input method to tap the code.

I just wanna express sometime the input method maybe incompatible.(输入法可能不兼容)

I have removed the solution code from your post. Please do not share working solutions in this forum.