I stick to a simple text + images in my signature, or sometimes just text that's modified to look pretty. The bottom stuff is usually just links to some of my other handles, and of course stuff I'm involved in. I always like to center it because it's neat when it's like that.
I mostly try different stuff every now and then and experiment with things, but my most iconic themes are the various Oshawott ones I've had over the years, especially because I have so many Oshawott avatars in my storage. Alongside my Oshawott themes, some other fave themes I've used include Froakie, Chandelure (I tend to do those every Halloween), Wooper, shiny Corsola (my fave best-looking shiny), alt-J and Foster The People (which happens to be what I have up at this moment).
I did sometimes use CSS back in 2013, but I always kept the same coding because I'm not very good with it and I found it a hassle at times. As for theme collabs, I have been in 5 of them so far, with the following themes:
#1 - Pansage (Green Pokemon)
#2 - Marlon (Gym Leaders)
#3 - Pikachu (SSB characters)
#4 - Marill (G/S/C/Pt Pokemon for new supporter system)
#5 - Oshawott (Baby + Base-stage Pokemon)
I also once did two mini-collabs, a Kyurem collab with Mid a.k.a. revere where I was White Kyurem and he was Black Kyurem. And both of us also did another mini-collab with Gunner a.k.a. arachnidsGrip featuring the Kalos starters. Mid was Chespin, Gunner was Fennekin and I was Froakie.