Change font size for all content in app

Ionic Framework ionic-v3
1

I have Settings in my app, the user can change the font size from radio button to make font bigger or smaller, how I can apply this idea

I tried different methods but always Fail

there any idea o any way

this html code

 <ion-item text-right>
      <ion-label color="dark" >notchange</ion-label>
      <ion-radio value="always" checked (ionSelect)="lineunchange()"></ion-radio>
    </ion-item>
    <ion-item text-right> 
      <ion-label color="dark">big</ion-label>
      <ion-radio value="big" (ionSelect)="linebig()" class="testclass">

      </ion-radio>   
    </ion-item>
    <ion-item text-right>
      <ion-label color="dark">small</ion-label>
      <ion-radio value="small" (ionSelect)="linesmall()"></ion-radio>
    </ion-item>