A Theme on your website creates your site look and feel.
Avoid Malicious Nulled Themes Nulled themes have had a wide impact on a website security. When choosing a theme for your website, avoid downloading themes from untrusted sources. Unfortunately there are many websites on the Web that distribute malicious themes. These themes are called “nulled” themes and they contain malicious code pre-installed. Only install themes from trusted sources.
The most popular source for themes is a official theme repository which you can find for example at WordPress.org. These are themes contributed by the community. They are open source and are generally reliable and infection free. When using a theme from a source you don’t recognize, try googling the site domain name in quotes and you may find reports indicating whether the site is reliable or engaged in a malware campaign.