There are 3 page background features that you can change in Microsoft Word. This article refers to the version of Word available from the Microsoft 365 subscription. Although these options are available in other versions their location on the ribbon may vary.
The page background features are available on the design tab and consist of watermarks, page colour and page borders.
Watermarks can consist of text or pictures which are shown on the background of every page. There are a range of options already created. This is supplemented by extras from office.com. Alternatively a custom watermark can be produced by changing a range of options such as text or pictures, font type, size, colour and layout. The watermark can also be removed from here too. A personalised one can be created, then selected and added to the watermark gallery to be accessed from the initial list for future use.
This changes the background colour of the whole page. Initially there are several theme and standard colour to choose from but more colours give a list of more standard colours or a custom option as required. In addition there is the option of using a fill effect. This could be shading from one colour to another, a texture, a pattern or even a picture.
Again there are some pre-created options to choose from although they can be customised using any style, colour and with you require and choosing which sides of the page these are to be applied to. There is also the choice of adding art which contains some more elaborate border options including picture borders. These can be great for posters.
So if you want to add some impact to your page have a look at some of these options and see what you think.