Forum Appearance
You can change your default view to always be threaded rather than linear. This can be set in your User CP-Edit Options thread view mode setting. Note that in threaded view, the thread listing appears first then the first post of the thread comes up in its entirety under that.

Different themes are available to choose for your own personal use on the forums. Themes determine the background, colors, and/or layout of information on your screen. You can set your theme by clicking on the button located at the bottom right side of the screen, after making your choice from the list to the left of the button.

In the default theme, an orange color for the file folder symbol on the main page listing Community Forums indicates forums which contain posts which you have not yet read. Once you have opened a particular forum containing new (to you) threads/posts, an arrow will show to the left of the thread title containing posts you have not read, and a boxed notice saying how many new posts there are in the thread will display to the right of the thread title.

When a thread has a new post added to it, it moves to the top of the relevant forum's thread listing. This makes it easier to tell which threads have had recent activity.