I’m not quite sure how this has happened, but 4-5 months ago after an update I'd noticed the gray background color on any program, ie, Dolphin, Gimp, browser or anything else has become too dark. I have not been able to figure out why or how it could have happened! Does anyone have run into the same problem, and is there anything I can do to lighten up the background color? I have messed around with all kinds of settings with no success!

The colors are completely configurable. System Settings -> Appearance -> Colors. Now here's the part that's a little tricky. There are several color schemes. The current one is the one currently highlighted. Click Edit... at the bottom to edit the colors. You can edit the colors of other schemes, but unless you select that scheme, the current colors won't change.

In the dialog that appears, there are a number of "Background" colors: View, Window, Button, Selection, and so on. You are probably looking for Window, but also maybe View, and possibly other bacakgrounds. Click the colored box to change a color and the Select Color dialog will appear. Once you change a color, the Save button will be selectable. I suggest saving to another name (e.g. I use "Breeze Dark - Mine"). Clicking apply after changing a color should immediately change the color on the screen. If you have doubt that it is working, select a really bright color (just remember what the color was so that you can reset it).

If clicking Apply has no effect, then you probably don't aren't editing the current color scheme. Close all of the dialogs including System Settings, and start over so that the current color scheme is selected. Editing the "Current" scheme shown at the top of the list does not work for changing the colors. In fact, clicking Edit... after selecting Current appears to bring up the wrong color scheme as it is not the Current color scheme, so stay away from Current.