In WordPress, a “theme” is not only a design template, but a complete package of design and functionality for a WP CMS installation. Within the WP administration, themes can be searched (with filters), downloaded and activated.
Each theme may have different features and functions, so the handling of your your WP site might also vary depending on which theme you use.
Portfolio Themes at WordPress
There are free themes with portfolio features available at https://wordpress.org/themes/search/portfolio/
Additionally, there are themes on the third-party market, which can be bought for prices from $10 to $75 approximately. Envato, a market leader in selling themes, templates, design and functions for the design industry, is offering over 7’700 WordPress themes and plugins at Themeforest. These themes must be uploaded manually, as they’re not available through the WordPress theme search.
Plugins do add functionalities to a WP site that WordPress itself doesn’t have. Like for instance adding forms to a site, or enlarging pictures on a click, embedding google fonts, or even complex ones like web builders. Such a plugin, the Divi Builder, will be at your disposition.
As with themes, there are plugins available for free through the WordPress plugin search, and more purchaseable at commercial sites like Envato and others.
Plugins should be used moderately, and tested before. Too many plugins may slow down the site, or even be incompatible with each other.