Themes are plugins, so you can use the plugin gallery to install and enable additional themes:
https://docs.pkp.sfu.ca/learning-ojs/en/settings-website#plugin-gallery
The sidebar slider you reference in your earlier post looks like a Custom Block which using javascript to implement a slider. To manage this without custom PHP code, you would need to:
- Install and enable the Custom Block Plugin
- Use the Custom Block Manager to create a new block with initial content
- Install and enable the Custom Header Pluguin
- Use the Custom Headers to load javascript to replace the initial block content with an image slider
- Position your new block in the sidebar with the Website Settings “Appearance”.
A similar example is described here: