Ionic 4 Theme variable doesn't work

I wrote the code below in the file of “variables.scss” or “global.scss”:

ion-searchbar {
–placeholder-color: white;

it should make the text of placeholder with white color, however, it doesn’t work.

But, if I write the code just in the page css file, it does work.

Does somebody know why?

Try to paste that same code in global.scss

ion-searchbar {
–placeholder-color: white!important;