I'm currently running IDE v1.8.2 which I have found has some bugs that have been cleared in 1.8.9.
http://forum.arduino.cc/index.php?topic=606855.msg4145091#msg4145091
I have two questions.
Will the new install a. disturb my existing .ino files/libraries in any way?, b. overwrite my current (dark theme) screen color settings?
TIA
dougp:
disturb my existing .ino files/libraries in any way?
The only way that will happen is if you have saved sketches or libraries under the Arduino IDE installation folder. Everything in that folder will be lost when you install a new version of the Arduino IDE. If you do have any of your own files saved in that folder, make sure to move them somewhere safe before installing a new version of the Arduino IDE.
dougp:
overwrite my current (dark theme) screen color settings?
In Arduino IDE 1.8.2, I believe the only place you could store Arduino IDE themes was in the lib/theme subfolder of the Arduino IDE installation folder. For this reason, you will lose your custom theme when you install a new version of the Arduino IDE. You'll be glad to know that, with newer IDE versions, it is now possible to install themes to the theme subfolder of the sketchbook folder. The sketchbook folder will not be affected by installing a new version of the Arduino IDE. So my recommendation is to copy your theme folder from the Arduino IDE installation folder to the sketchbook folder before you install the new version of the Arduino IDE. You can find the location of the sketchbook folder in the Arduino IDE at File > Preferences > Sketchbook location.
Success! Thanks for the assist.
You're welcome. I'm glad if I was able to be of assistance. I hope the new IDE version will be good to you!
Per
pert:
I hope the new IDE version will be good to you!
Per
Alas, no. It mostly worked, then I saw when I opened a file that numeric literals, which had been yellow under the previous dark theme are now black. Likewise [],(), and {}, which had been an off-white. I've edited the theme file, now in the sketch folder, and pasted into the Arduino lib folder but no joy. I've been restarting the IDE figuring it loads things like themes only at startup but this has no effect.
Specifically, I altered these two color settings with no effect.
# TEXT - LITERALS
# constants & datatypes
editor.literal1.style = #006699,plain
# p5 built in variables: e.g. mouseX, width, pixels
editor.literal2.style = #F8F53B,plain
editor.variable.style = #F8F53B,plain
In the standard theme file they are: #00979C,plain.
So, first off, am I changing the correct values?
Edit: I just found this: GitHub - jeffThompson/DarkArduinoTheme: A dark theme for the Arduino IDE – no longer maintained :(
Seems there are some new wrinkles to navigate. >:(
dougp:
I've edited the theme file, now in the sketch folder, and pasted into the Arduino lib folder
The folder structure should look like this:
{sketchbook folder}
|_theme
|_buttons.png, etc.
|_theme.txt
|_syntax
|_default.xml
dougp:
I've been restarting the IDE figuring it loads things like themes only at startup
That's correct.
dougp:
Specifically, I altered these two color settings with no effect.
Are any of your modified theme settings having an effect?
dougp:
Edit: I just found this: GitHub - jeffThompson/DarkArduinoTheme: A dark theme for the Arduino IDE – no longer maintained :(Seems there are some new wrinkles to navigate. >:(
There were some changes to the Arduino IDE's theme system in Arduino IDE 1.6.5, but I'm not aware of any since then.
pert:
The folder structure should look like this:{sketchbook folder}
|_theme
|_buttons.png, etc.
|_theme.txt
|_syntax
|_default.xml
My sketch folder does not have such a folder within. Are you saying I have to put it there manually?
We *are *talking about the folder with the sketches I've created, the libraries and such, correct?
pert:
Are any of your modified theme settings having an effect?
Some. Here's a snipped view. The blue and green and orange/red carried over from the previous dark theme - which I did not alter. The parens after setup and the baud rate for serial.begin are there, even if they can't be seen - it's a black on dark gray thing.
I did find in my search an option for a built-in dark theme 
but I could make it work.
Edit: I was able to relocate the built-in dark theme - craftily hidden right at the top of the program files (x86)\Arduino\lib\theme\syntax folder - and activate it. Those devils!
There are two files in there, 'default' and 'dark'. I just renamed them to 'default orig' and 'default' respectively.
dougp:
My sketch folder does not have such a folder within. Are you saying I have to put it there manually?
If you want to install a custom Arduino IDE theme to your sketchbook, yes you need to put it there manually. The theme folder won't be there already, you need to create it.
Or, if you're happy with the default theme of the Arduino IDE, you don't need to do any of that.
dougp:
We *are * talking about the folder with the sketches I've created, the libraries and such, correct?
The sketchbook folder location is shown in the Arduino IDE at File > Preferences > Sketchbook location.
dougp:
So, first off, am I changing the correct values?
No. Since Arduino IDE 1.6.5, some of the theme properties are now set in theme/syntax/default.xml.
dougp:
numeric literals, which had been yellow under the previous dark theme are now black.
That is controlled by the LITERAL_NUMBER_DECIMAL_* properties in default.xml.
dougp:
Likewise [],(), and {}
That is controlled by the SEPARATOR property in default.xml.
dougp:
Specifically, I altered these two color settings with no effect.# TEXT - LITERALS
editor.literal1.style = #006699,plain
editor.literal2.style = #F8F53B,plain
editor.variable.style = #F8F53B,plain
In the standard theme file they are: #00979C,plain.
Those theme properties no longer have any effect.
I have attempted to document the Arduino IDE's theme system here:
That repository also contains a dummy library that can be used for testing themes. It can get quite tricky to find colors that provide good contrast against each other for both highlighted and non-highlighted text.
pert:
If you want to install a custom Arduino IDE theme to your sketchbook, yes you need to put it there manually. The theme folder won't be there already, you need to create it.Or, if you're happy with the default theme of the Arduino IDE, you don't need to do any of that.
The built-in dark theme, as far as I can tell, is in appearance just like the previous version. Happily I can now choose either theme by simply renaming and avoid lots of aggravation.
I feel like I ran around the lake to get from my front door to the back door. Thanks for your forbearance!