isDesktopClassic() {
        return UITheme.getUITheme() === "Theme3";
    }

@AuraEnabled (cacheable=true)
public static boolean getIsUserInClassic(){

    //Run class to see if the user is in Classic or Lightning
    String themeDisplayed = UserInfo.getUiThemeDisplayed();

    //Boolean for Classic
    boolean displayClassic = false;

    //Populate checkbox for handiness
    if (themeDisplayed == 'Theme3'){
        displayClassic = true;
    }

    return displayClassic;

}

import getIsUserInClassic from '@salesforce/apex/HelperClass.getIsUserInClassic';

export default class sampleClass extends NavigationMixin(LightningElement) {

@track isUserInClassic;

connectedCallback(){

        getIsUserInClassic()
        .then(result => {
            this.isUserInClassic = result;
            this.error = undefined;
        }).catch(error =>{
            this.error = error;
            this.isUserInClassic = undefined;
        })

console.log(isUserInClassic);

}