You can customise the main sections of the UI as follows.
By right clicking in the toolbar area, you can add and remove toolbars. Holding SHIFT whilst adding a toolbar will insert a new instance of it. Rightclicking in the band area of the toolbar may bring up additional actions for that toolbar (such as the spectrum analyser).
You can change the appearance of the buttons through the buttons customisation dialog accessed through the buttons context menu.
You can change the colours used in the spectrum analyser from the context menu displayed when you right click on the spectrum analyser.
write section - live editing and layout preferences page.
You can change the colours used in the built-in panels from the Colours and fonts preferences page.
Customising of the playlist views is done through the Playlist view configuration pages. Customising the colour for on a per-column basis is done via a style script. This is either the global style script, or the column style script, depending on whether you have activated a custom style script for that column. Adjusting the contents of the column is done via the column's display script. Adjusting the font, and exposed background colour is done via separate settings.
See the aforementioned configuration pages for more details on adjusting these settings.
You can adjust the contents and shown panes of the status bar from the Status bar configuration page.
The text of the main window title can be adjusted through the main configuration page.
The text of the notification area icon's tooltip and the icon itself can be changed through the notification area configuration page.