If you have updated your theme to version 4.8 or above, you will remain in Legacy (not-responsive) mode after the update is complete, and must activate the Modern (responsive) skin. This will essentially open your existing theme with a new stylesheet, optimized for both desktop and mobile viewing.
To enable the Modern (responsive) skin, we should first make a Save of our Current Customizations, by going to Theme Options > Choose Theme > Save Current Customization. Its good practice to make a Save each time you do any significant changes to your theme's style.
Once you've saved your settings, proceed to Theme Options > Customize Theme > Global > Theme Skin. Here you will switch from Legacy (not-responsive) to Modern (responsive).
*Note: Starting with version 4.8, and new preset that you activate for the first time will automatically use the Modern (responsive) skin.