You will need to install a user script manager extension to install this script.
Hover over a topic and a trash icon will appear; clicking it will hide the topic, and it will be hidden on subsequent visits to the forum it's in and on the Unread Content page.
On the Unread Content page, a trash icon will also appear next to the forum a topic is in - clicking it will hide all topics in that forum when browsing Unread Content.
If you've accidentally ignored the wrong thing, or if you realise you want to read about insert name of collectible physical object or insert name of football club more than you thought, open your script manager's user script commands menu and click "Show ignored topics".
Ignored topics will be displayed with a fetching salmon highlight and clicking the appropriate trash icon (you may have ignored a topic individually, by forum or both) will unignore them. You can then re-hide ignored topics using the same menu item.
When using the forum's Fluid view, a collapse/expand button will also be added which allows you to hide the sidebar once you're done picking folders.