When a party is 'themed', I prefer to play off the theme in an abstract sense, rather than a literal interpretation.
In the dog days of summer party we're discussing this week, rather than make everything dog-related, I might choose to play off Otto the dalmation's spots. So we might choose black, white or red lanterns (abstract 'spots') for decor, polka-dot wrapping paper for favors or gifts, and use 'spotted' ribbon as adornment. If you prefer more heavily themed decor, you can intersperse items such as dog bone placemats (if the party is seated) or use (new, clean!) dog bowls as serving containers or centerpiece holders for flowers. Just make sure you don't go overboard! While over-the-top can sometimes be fun for parties, it can also be a tad nauseating to your guests.
Food is another fun element in any party. If you plan on catering or picking up pre-made food, I would suggest adding at least one homemade touch (like a dessert), to make the party feel more personal and thought-out.
I encourage you to pick and choose what things you want to put more effort into, and what things you want to delegate. If you put too much time and energy into party planning, you're much less likely to have another one anytime soon, since the memory of your most recent fête will be exhausting! If you enjoy cooking- don't make homemade invitations or favors. If you love the decorating part of party-planning, choose to get some or all items catered. Parties are supposed to be fun! Not just for your guests, but for you. And just like a wedding, the party itself is a lot shorter then the planning part of it...so you want to make it as enjoyable as possible.
Above are some of my picks for Jack's dog days of summer, dalmation-inspired birthday party. As you can see, I've chosen items that both personalize the party (dog cupcakes, cookie cutters) and items that are more abstract (lanterns, ribbon). The abstract items can be used for another occasion as well, so it's a brilliant way to get more use out of your party items.
From top: red dot wrapping paper from paper source, cupcakes from flikr (with the addition of some icing 'spots' these could easily translate to a dalmation), red lantern from luna bazaar, lollipop from hammonds, red circle stickers from paper source, dog house and bone cookie cutters from cookiecutter.com, bone mats from oriental trading company, and ribbons from repro depot.