@hequaye the plugin suggested by @HAWK above is not available on our hosting (it’s unofficial and at this point it is quite outdated). The Custom Layouts plugin is also not available in our hosting.
Your best bet for a sidebar is to look in the #theme category and search for “sidebar”. Here are a few candidates:
- Discourse Category Sidebars
- Discourse Tag Sidebars
- Atlas, a simple blog-styled theme with an optional sidebar
- Topic list sidebar navigation
Some of these have the sidebar on the right, but it can be moved to the left with a little bit of CSS in most cases. If you need more help with your site specifically, feel free to write a message to @team.