Change everything you want. Set a background image, change the button text, background color and add an attractive text on it.
Tip: Check your Google Analytics Avg. Time on Page section before deciding after how many minutes to appear the pop-up on the website. If the value is 5 minutes, multiply it with 0.3 (30% of the visit) and thus you obtain the optimal time for the pop-up display.
These pop-up newsletter subscribe forms with optimized time to show up will get you more subscribers.
And don't forget that you can tag each subscriber (add to segment) and start a welcome emails automation once they subscribe. That’s also another email marketing best practice you can make use of.