Picking a theme for your home can be fairly easy because its purely down to your personal preference, although the difficult part is applying the theme to your home. Common themes for homes include Asian/Japanese, retro and rustic style rooms.
Beside what colours your going to use an important factor of your theme is what furniture you will use because the furniture can make or break the theme of a room. Depending on the theme you have choosen you will be able to use new or used furniture, for example if you chose a retro theme you can buy new retro style furniture or pieces from the era. If you want a rustic theme you could take a fairly inexpensive chair sand it down apply a rustic style varnish and your chair instantly fits in with your theme.
You then need to decide if you are going to have the theme running through the whole of your house or just in one room for example your living room or bedroom. You may prefer to have a 50's retro style decoration in your living room and dining room, while keeping your kitchen area much more modern and practical. If you want to have different themes in your home for different rooms try not to combine them or pick too different a theme as it can make your home look broken up and the flow of the house can feel disrupted.
Also when themeing any of your rooms if you choose an extreme theme such as space or the jungle then you may need to make sure you tone the decor down if you come to sell the house. This is because it could put off potential buyers if there is an intense theme making it difficult to imagine the room with their own design.
Of course, you may not want a specific theme such as an Oriental or modern theme, you may simply want to use colours from a particular theme. This is also much simpler, as you probably won't need to purchase any additional furniture.
If you change your mind regularly or feel you may grow bored of a particular theme easily, when you're creating your room you may want to just have features such as pieces of furniture or art work related to the theme, this way the design can be easily changed.