strange theme on Galaxy S4 my little brother has played on my Galaxy S4. I don't know what he did. Now it looks strange. 

Does anyone know how do i fix it.

Thanks in advance.
Since this is a Samsung Galaxy, evident from the screenshot, I would look first to the "Home Screen Mode" in the Device section of Settings.

Settings > Device > Home Screen Mode

See if the setting is not "Starter Mode", which gives a simplified home screen with large icons.

If so, switch the setting back to "Standard Mode" and click Apply button. You may have to bring your icons back manually when you do so, to configure the home screen to your liking again.