Believe it or not, this game was a pretty much a first for me.  While it is really the fourth game I made (Game boy Hero I & II were rhythm games, and I'm pretty much done with a board game that I haven't published yet), this was my first platformer, my first time working with large scale animation, my first time creating actual backgrounds (and designing them for parallax scrolling), and my first time making sprites that weren't just 16x16 people.

As for the art, the mixed styles wasn't really intentional, it just sort of happened.  I actually was a little concerned about the inconsistency of it all, but it all did a great job of telling the story and came out great so I just ran with it.  I never really thought about it, but yes, it does lend itself to a bootleg kind of feel which I think is pretty appropriate for a game whose whole concept was pretty much a joke to me.  But while I made it as a sort of joke, at the same time I do feel that it is a fitting tribute to Warren Zevon and that he'd probably have a good laugh at it if he were still alive today.

Anyways, I'm really glad you like it.  I'm currently working on another game for a different jam that will follow in similar footsteps.

So for the splash screens all I do is take an image, then convert it down to only a few colors, sometimes 4, other times more, depends on how it looks.  If its more than 4, I then need to choose which colors get changed to which, and sometimes I may divide them in half, so a color might be light green on a head but dark green on a body, or whatever.  And then I need to make adjustments here are there to make it look better, working on a pixel at a time.  It can take some time, but I enjoy it.

As for the backgrounds, I tend to take inspiration from places and run with it.  For example, the Moscow background is literally the splash screen from Tetris on Game Boy, then I just expanded it a bit keeping the same style and throwing in a few more of Stalin's palaces.  Havana came from a picture of Havana, I saw the same patterns on every building, just in different heights and colors, so I simply recreated that pattern and then adjusted colors and heights of the buildings.  As for Honduras, I worked with actual pictures of palm trees and Mayan pyramids.

With splash screens and backgrounds you also have the issue of tile limits, and I used the DMG limits, not GBC as I intended for it to be GB game.  So you gotta be smart and cut and paste tiles where it won't be noticeable to get it within the limits.  Ultimately I had to make it a GBC game because it just ran too slowly on the DMG that it wasn't fun.

The sprites, also pictures, cartoon pictures, or even sprite pictures that I then ran thru the same processes.  Since they are on a small scale and only have three colors you really gotta put some effort into it to make them look good, but since they're small it doesn't take all that long.

I actually studies architecture so took a lot of classes on using phot editting and 3D modeling tools in grad school, and all of that translates really well into making Game Boy art.  Well maybe not the 3D modeling.

Hope this helps!

I got really good at making music from my Game Boy Hero games, and I still have probably 30 songs I want to add to Game Boy Hero II.  And I think the endless runner concept worked amazingly well given it is a Game Boy game.  Sadly its a bit too much for the DMG to handle so I had to make it a GBC game, but it was all designed to be played on the DMG.  But the animations is where I spent the bulk of my development time, it became a lot of fun to experiment with how far I could push the Game Boy, and where its limits truly are.  I can tell you that the rom plays flawlessly on my Analogue Pocket.

The cool thing is, I think taking what I learned about large scale animation from this game, and creating a simplified Game Boy version of an endless runner, I think I can probably make a very legitimate Temple Run port for the GBC, and I plan on doing so in the not too distant future.