When it comes to computer (os) themes, Are you using the default, other pre-made theme, or did you create your own theme?

Create my own.  

I was into that "Big Time" when I ran Windows, because I got sick
of everything being MicroSoft Blue. Now all I've done is change my
wallpaper and the default browser to Opera.

I use customized purple background and green type:

 Like this right here. 

All I changed on the computer was the backgroun. My phone is just like I got it.

I didn't bother. It's not on the desktop screen for long and I'm not worried about the default blue.

I once put a landscape photo I'd taken, on my works computer, once I'd discovered where its photo selection lived, but that was as far as I'd ever gone with such things. 

In the days of cathode-ray tube monitors one had to run screen-savers when the computer was resting, so as not to imprint static images in the phosphor, and there were some wonderful ones available. My favourites were "Star Field" (which seemed inspired by the view from the Star-ship Enterprise in Startrek) and "Beziers", which were constantly changing sets of coloured mathematical loops. Another popular one was "Pipes", which grew screen-fuls of pictures of pipework like something in an oil-refinery. You could modify the colours and operating speeds of many of these.  

Incidentally my first computer was an Amstrad PCW9512, whose accompanying disc of applications included a Digital Research LOGO compiler. One of the examples in the teaching manual for that language apparently used an operating-system value called 'parity' to help it produce a never-ending trace very like 'Pipes', but just as a single white line zig-zagging around the screen. The manual said it had no practical use but filled the screen nicely! 

All I did was change the task bar color and the Desktop picture. I didn't bother with anything else much. Changed some of the icons looks on my security suite also.

It is as it came from the manufacturer.  I'm not big on fussing around with things that are unimportant to me.  All it has to be is legible and it is.

i created some themes for google chrome :D https://www.themebeta.com/chrome/user/426864
(there is still one, that im trying to perfect. the pink/hearts one but it will get there :P).. 

I used to tweak everything, especially when I was new to PCs.  But now I pretty much leave everything alone except my desktop wallpaper.  

Sometimes at work, if you were dealing with anything sensitive (company secrets or personnel matters, say) you needed to lock the computer off at times, so a pretty picture or screen-saver brightened up the office a little. At home you aren't really looking at anything other than the particular application you are using. 

My own desk-top background image at work for a while, was a landscape photo I'd taken.

Talking of screen-savers, a few days ago I encountered a video that would have good for this. I wanted to learn to make an eye-splice in ordinary rope, and the web-site manual I found happens to present a series of step-by-step images using a rope with its strands coloured to help illustrate the instructions. There are no fingers and hands in the pictures; just rope on a neutral background. If you let it run as a video the rope elegantly splices itself! It's slightly startling at first, then rather soothing to watch just as an image - given a random location-step function, it would have been quite neat as a screen-saver.