You may change the look of your office using the Decor menu, which can be accessed in the Office itself. It is the first button Decor (内装) in the menu on the bottom of the screen. When this button is clicked, a different set of buttons appears at the bottom of the screen, each of which denote different categories of furniture.
Furniture you already own has a stamp next to it. Red labels indicate furniture which is in use. Yellow labels Limited Time (期間限定) indicate seasonal furniture, which are available only for a limited time like the name suggests. Some of these seasonal items will be available again when the same season furniture items roll out a year later.
You may buy furniture using the proper amount of coins (given through events and completing research missions) and decor licenses (given through events).
Furniture (道具), large items like book shelves and cabinets usually near or against the back wall on the right.
Wall (壁), wallpapers covering the office.
Window (窓), mostly items of various kind of windows, verandas, doors and so on the left side of the office showing the Courtyard (or other locations).
Interior (装飾), various items from paintings and portraits to small shelves, cabinets and plants. Basically smaller items fitting on the left corner of the back wall.
Ceiling (天井), like the name suggests, various ceiling styles.
Desk (机), mostly different kind of desks or tables, but sofas, kotatsu and other large items can be also found under this category. They are placed around the middle of the office.
Floor (床), various floorings.