Ok, Here is my config, and what I have done so far.
Environment: Linux Mint 19.3. I installed Geany and Geany-Plugins from the Software Manager, and both are version 1.32. I recognize that this is a somewhat older version, but initially this was an experiment. Later I may add the PPA repository and install the current stable release. Mint 20.3 should be released around Christmastime, and I plan to reformat and reload that release, which has Geany 1.36 in its Software Manager.
Next, I downloaded the Themes from https://github.com/codebrainz/geany-themes (be sure to hit the Green button to download the .zip file). I unpacked the .zip, and copied the contents of the colorschemes directory into ~/.config/geany/colorschemes. This makes the additional themes available under View/Change Color Scheme. I then opened a variety of documents and source code files, and looked at how they appeared under different (dark) schemes. The best 3 to my eyes were Black, Vibrant, and Ralesk's MC. After more checking, particularly in the Syntax Highlighting, I settled on Ralesk's MC.
The Documentation for the Theme and color settings is described in the Manual. Look for the "Filetype definition files" section. Some settings are in the filetypes.common file, which on my system is located in /usr/share/geany/filedefs. If you decide to alter any of these settings, first copy this file to ~/.config/geany/filedefs and do the edits on that copy, rather than altering the master copy in /usr/share/geany/...
At this point I started editing and experimenting with changes to the ~/.config/geany/colorschemes/mc.conf file (I have a fresh copy elsewhere, of course). I'm not nearly done with this process yet, so I can't report any results yet, but that's where I am so far. I have succeeded in affecting , for example, the colorization of Comments, and a few other changes. But some things don't seem to work as expected - need more trials.
Regarding the command-line parameters issue, I found that clicking Build / Set Build commands, then appending the parameters to the Command field of the 1st line under Execute commands allows the parameter string to be passed to the current script, when pressing F5.
Hope this helps someone. I may post again when I am further along with the Theme work.
Alan Becker