Sterling, I'm waving at you as we pass. My wardrobe was much too light-hearted (unsure of fun) for my present needs. Of course our styles will be different, but maybe if I describe the elements of my clothes, you can find something adaptable.
1) Color. I think I remember you saying your wardrobe is a sea of black and navy? Light-hearted colors might be your version of pinks (fuchsia!), yellows, reds, greens, brights or even some pastels, all which can be worn with your neutrals! Find that fuchsia! (If you do pre-owned, you can search by color and size on ebay)
2) Prints. Florals, gingham, polka dots, all read light-hearted to me. (I know not everyone is a print person. But my wardrobe has at times looked like a garden--leaving me with all prints and thus nothing to wear.) The green floral pencil skirt Angie featured today is an example of what I would find as light-hearted.
3) Silhouette--perhaps more flow and/or swish? Kind of the opposite of a strongly tailored style.
4) Details such as trimmings, lace, embroidery (can even be tonal for lower contrast).
5) Sandals and pink polished toenails.
Anyway, just some ideas from a formerly light-hearted person.
I'm very excited for you, and I hope you keep us updated on your changes...or even if you decide things are fine as they are.