You are designing a website or a multi-window system; in order to help simplify the user experience, the superficial design and methods of navigation must be consistent across all the different sections of the user interface.
When you are dealing with a complex user interface it is essential to implement a visual framework, as the user should be able to apply knowledge of how the website or system appears and behaves from page to page or window to window.
Consistent design is essential to the user experience; if superficial aspects of the design and/or methods of interaction change when the user arrives at a new page or window, they must learn new layouts and new representations for the same actions, which is needlessly time-consuming and cognitively burdensome.
Colors, fonts, layouts, and styles might seem like simple aspects of your design, but they all provide useful information to the user as they move through the different contents of a web or system design. A quick glance at the top of a webpage can be enough to determine where you are and the various navigation points on a site. However, if items are in different points across the pages of a website or windows in a system, the user is instantly thrown off; disorienting them and forcing them to relocate the user interface elements of current interest.
Furthermore, changes in the user interface arrest the user’s attention; if every new page greets the user with a new style, layout, etc their eyes will be drawn to these unnecessary superficial changes, rather than directing the user to the most meaningful part of the display (i.e. the contents). Therefore, a consistent visual framework directs the user’s attention and helps them filter out the rest of the user interface.
Aside from assisting the user, strong visual frameworks help designers establish a clear ‘look’, which helps separate your brand from all others.
If there is an unhelpful element in your design a consistent visual framework will multiply the problem by however many pages or windows there are in the user interface. For example, if you have chosen colors that clash in the display, they will cause problems for the user no matter what page or window they are on. Therefore, the consistent bad design could be just as problematic as an inconsistent user interface. For this reason, you must ensure the superficial design supports the user experience overall, as well as focussing on the individual elements that comprise the user experience.