Change the Look of Your Mailbox in Outlook on the Web
These all feature will help the user to customize their inbox on outlook on the web as per their comfort and it makes the web interface more attractive.

In office365 outlook on the web, you can easily change the appearance of the message list and reading pane, so it looks and works the way you want.
Change how your email is sorted:
1. At the top of the mailbox list, select Filter.
2. Select Sort by and choose the option that how you want to sort your email.
Open the Settings pane
To find the most frequently used Mail settings, select Settings Settings at the top of the page.
To change settings that are not included in the Settings pane, select View all Outlook settings at the bottom of the Settings pane.
Theme
Under Theme, choose from one of the themes displayed or you can also select View all to choose from more options.
Dark mode
Dark mode switches the color scheme from light background to a dark one. Turning on dark mode can reduce eye stress in low-light environments.
Focused Inbox
When Focused Inbox is on, it sorts messages in your Inbox into Focused and Other tabs to help you focus on what matters most. If you want to see all your email messages in one place instead of having them sorted into Focused and Other, select the toggle to turn it off.
Note: Focused Inbox works only in your Inbox. It doesn't sort messages in other folders.
Desktop notifications
Desktop notifications are to let you know when you have received a new message or have an upcoming calendar event, even when your browser window is minimized. You might need to enable notifications in your browser to use desktop notifications in Outlook.
Display density
Display density will help you to adjust the spacing between messages in your message list. Choose the Full view to see the sender’s images, attachments, and message preview text (sender, subject, and part of the first line of text). Medium will show only message preview text with some spacing between messages, and Compact shows only message preview text with minimal spacing between messages.
Conversation view
Conversation view groups your messages by conversation. You can see the conversations with the Newest messages on top or Newest messages on the bottom. If you prefer, you can turn the Conversation view Off.
Reading pane
You customize the Reading pane by selecting Show on the right or Show on the bottom. If you do not want to use the reading pane setting, select Hide.