Colors - The Simplest Way
Some people choose their theme to be a color. The color can be any color that appeals to the eye, but common choices are red, blue, or green. Sometimes, the decor is more than one color, like "blue and white" or "black and white" for the unique look of the two colors in combination. When a color is a kitchen decor theme, accessories and decor are easy to do.
Keep the same color(s) and use it in several places - paint, curtains, tablecloth, hand towels, and spoon rest, for example. Don't try to use it everywhere, or it will be muddy and the effect diluted. It is much better to have the room be white and use the color as an accent than to try to use the color everywhere.
Style - Furniture, Too!
If you don't mind including changing the furniture if necessary, your kitchen decor theme and accessories can be influenced by your furniture style. For example, antique furniture (or furniture with antique styling) lends a different feel to a kitchen than one where there is European styled furniture and stainless steel appliances. Be sure that the look you are going for is one you really like, as this style is hard and expensive to change!
Getting Fruity?
Some people like to decorate their kitchen with a fruit theme. Apples are common for this, as are grapes. The nice thing about apples (or grapes) are that they can incorporate many colors because of the fruit - green and red - and also go well with other colors, like brown. If your kitchen decor theme and accessories are fruit, look for a wallpaper strip (to use around the top edge of the kitchen) that includes your chosen fruit.
Or art for the walls of the dining area with the fruit. Look for a vase or other container decorated with your fruit to hold long-handled utensils on the counter, or place mats that have the fruit on them. If you chose grapes, a popular extension is to incorporate wine into the theme, with other wine products visible in the kitchen.
Oh the Places You Go...
Another alternative is to have your kitchen decor theme and accessories revolve around a place. Maybe you went to Paris, France or Venice, Italy - why not decorate with that place in mind? Have artwork that reminds you of your trip, or hang items from your trip on the walls. If you haven't yet gone, but dream of someday taking a trip there, that's ok, too. Decorate in anticipation of the trip!