Fix checkbox transparency

assets/scss/variables/_variable-material.scss

@@ -297,7 +297,7 @@ $progress-circular-spinner-width-sm:   0.15rem !default;
 // Based on https://material.io/components/selection-controls/
 
 //$selection-control-border-radius:               50% !default; // added djibe
-$selection-control-checkbox-icon:               url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" height="24px" viewBox="0 0 24 24" width="24px"><path d="M0 0h24v24H0z" fill="none"/><path d="M19 5v14H5V5h14m0-2H5c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h14c1.1 0 2-.9 2-2V5c0-1.1-.9-2-2-2z"/></svg>') !default;
+$selection-control-checkbox-icon:               url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" height="24px" viewBox="0 0 24 24" width="24px" opacity="#{$black-secondary-opacity}"><path d="M0 0h24v24H0z" fill="none"/><path d="M19 5v14H5V5h14m0-2H5c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h14c1.1 0 2-.9 2-2V5c0-1.1-.9-2-2-2z"/></svg>') !default;
 $selection-control-checkbox-icon-checked:       url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" height="24px" viewBox="0 0 24 24" width="24px" fill="#{selection-theme-color()}"><path d="M0 0h24v24H0z" fill="none"/><path d="M19 3H5c-1.11 0-2 .9-2 2v14c0 1.1.89 2 2 2h14c1.11 0 2-.9 2-2V5c0-1.1-.89-2-2-2zm-9 14l-5-5 1.41-1.41L10 14.17l7.59-7.59L19 8l-9 9z"/></svg>') !default;
 $selection-control-checkbox-icon-indeterminate: url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" enable-background="new 0 0 24 24" height="24px" viewBox="0 0 24 24" width="24px"><g><rect fill="none" height="24" width="24"/></g><g><g><g><path d="M19,3H5C3.9,3,3,3.9,3,5v14c0,1.1,0.9,2,2,2h14c1.1,0,2-0.9,2-2V5C21,3.9,20.1,3,19,3z M17,13H7v-2h10V13z"/></g></g></g></svg>') !default;
 $selection-control-color:                       $black-secondary !default;