Office 365 (Outlook on the Web) - Settings, Display, and Message Options
Display Settings
To access the various display options such as Theme, Dark Mode, Focused Inbox, Display Density, and Conversation View, click on the Options icon in the upper-right. For more information on these options, select the "i" to the right of the option. 
Outlook Mail Settings
To see additional settings such as Compose and Reply (signature & formatting), Attachments, Rules, Junk Email, Message Handling, and Auto Replies (out of office, vacation, etc.), select View All Outlook Settings at the bottom of the Options menu as pictured above. Once selected, the following menu will appear:
Message/Mail List Options
The Message/Mail list shows the messages in the current folder that is selected on the left.
Select the down arrow 
to expand your Favorites, Folders, and Cabinet lists or select the up arrow to 
to collapse and hide them. Select any folder in the list, such as Inbox to view its contents. The messages displayed can be managed by selecting a Filter or by using Search.
The message list contains tools that allow you take care of some tasks.
- When you hover over a message in your mail list, a set of icons appears. The options are Delete, Mark as Read/Unread, Flag it, or Pin to Top. Other icons show you which messages have attachments, importance (high or low), and how many unread items there are in a conversation.
- By right-clicking on a message, you can see a full menu of options, such Categorize, Move, Mark as Junk, or Create a Rule. On a touch screen, use tap and hold to view the message options.
- By selecting the triangle to the left of a message, you can expand a conversation to see all messages in that conversation.