Customizing a theme is done through the preferences page inside of the "Appearance" tab. A preview of each theme available is shown below.
Date format and timezone
Customizing the date format and timezone is done through the preferences page inside the "Appearance" tab. You able to select the date format and timezone according to your needs.
User page and avatar customization
Customizing a user page can be done by going to your user page and clicking the edit button. The user page can be accessed by clicking your name in the top right or in a linked area. You can edit the text on the page like a regular page and save when you are done.
How do I modify my userbox?
Modifying the userbox is also a straight forward process. You click on the userbox itself in the visual editor and click edit template.
Step 2: You can modify the text as needed. For example, to change the displayed avatar, you can upload a file and then put the file name inside of there.
How do I have an avatar by my name in logs?
You can set this avatar in preferences by clicking upload avatar.
You have to set the avatar on the user page manually. The avatar featured inside of preferences only shows up right beside your name in a log. The instructions on how to set it as the avatar in your userbox are featured in the above section.
How do I edit another user page?
You are only allowed to edit your own user page. Attempting to edit another user page will trigger the abuse filter and cause the edit to be blocked. However, you are allowed to add a message to their talk page if you want to get in contact with the user.