To customize this theme, you have to use both the Dashboard and Customizer. Like other themes from Catch Themes, you have to create the posts for testimonials, services, portfolio and featured content. And then, go to the Customizer, you will see abundant custom features, from the minor to the major and advanced ones.
This theme doesn't force you to set up a static front page to show the special features like featured post slider, featured content, hero content, services,... So you don't have to do many complicated steps. Like other Catch Theme's free theme, Clean Portfolio gives you sufficient choices for these special parts like hiding, showing it on the front page or entire page and so on. The number of testimonials and portfolios is unlimited while that of services, portfolio and featured content is up to 20. That's such generous numbers, right?
Featured Slider is a bit different when it gives you a demo slider like this. However, the sliders are only the images without any URL inserted so I think they're for decorative purposes only.

Demo slider-
Luckily, you have the right to add pages here. I think you should use this way to show your integral pages.

Customizing featured post sliders
The number of different layouts for the whole site is still limited, but you can still use them to redesign your page to some extent. They can be all rendered properly.

Layout Options
In the Theme Options section, there are also many more features for you to meticulously style your page like the breadcrumb, pagination type, scroll up, search options.
Next, you shouldn't miss the common changeable feature like color, background image or header media as they help your page more creative and beautiful.
The tone of color is limitless, but there are only 2 sections so I don't think they can considerably change the appearance of your website. That's why a background image is given to help you prettify it.

Customizing color
Like many other themes, an image can be selected and cropped to any size for the header image area. Moreover, a full-screen video can be added very simply by inserting a Youtube URL or uploading a video.

Customizing header media
By creating a custom social menu and designate it to the Social menu location, you're unrestricted to add any custom URL. The icons will appear on the header as long as your social networking site is the popular one.

Customizing Social menu
The last words, it has 3 footer columns and sidebar for you to add all the common widget from WordPress. All these widgets can be displayed nicely in the right way.