That color is set by the OS. You used to be able to change the system colors using the old control panel color settings but the ding bats at Microsoft removed access to that interface.
You need to use a tool that can address the old windows color settings. One tool I use is called Classic Color Panel
. You can use it to change the 'AppWorkspace' color in order to change the background color you are looking to change. You can change that one color, click Apply it will ask you to log off and that's it. You log back in and that color should be changed.
I have gone thru and found most of the mappings for Datacad using Classic Color Panel. Below is a list of the different colors I have found. One thing to note, this will change the colors used in other older programs as well so I would suggest making a list of any crucial older programs you use and making one color change at a time if you are going to change the colors drastically. The program also has a Default option to reset everything back to their Windows 10 default colors which is very handy to jump between different color schemes or if you get too crazy.
I stay in my dark mode most and then I can default back if I run into an app that I need default colors for, but I haven't switched back in sometime now as Datacad is really the only program that I use regularly that is affected. When applying a color change the app will need to log you out and then you can log back in to see your new color scheme.
It's not a perfect recolor but it gets the job done and helps with the blinding brightness working at night.
You can download the attached zip file below and double click the contained registry file to load this Datacad Dark Mode presets as shown above before running Classic Color Panel to load the colors shown here. You can save your own schemes as REG files to quickly manage multiple schemes for your different needs or moods.
Datacad Color Mappings
ButtonText: Datacad Toolbar Arrows
Window Text: Text used in window (Common Item Dialogs Like File, Open and File, Save)
Background: Windows Desktop if Solid Color is set
Button Face: Datacad's Toolbars & Menus
ButtonHighlight: Hover outline of Datacad buttons & menus (lower right)
ButtonShadow: Hover outline of Datacad buttons & menus (upper left)
Highlight: Hover color in dropdowns (Status Panel, single menu items in The Symbol Browser, Common Item Dialogs, File\Open and File\Save)
Menu: Any Datacad based menus (non-windows)
MenuHilight: Hover color in menus or menus with more options.
ThreeDDarkShadow: Hover outline of Tooltips (lower right)
Window: Field color in windows, dropdown buttons (Status Panel, Symbol Browser, Common Item Dialogs, File\Open and File\Save)
Window Frame: Used in a few place at window types of borders (mainly see between main menu at top of screen and drawing board area)
App Workspace: Space behind drawing boards (all drawings closed)
HotTrackingColor: Used for things like doing a drag selection on your desktop to select multiple icons at once.
Entries not seen / used / legacy
The other option is using a full system theming engine like Stardock's Window Blinds.
That is a whole other ball of worms so I'll just drop a link and you can see if that is something you might be interest in. Most of their themes are too busy or crazy but some are ok. I am currently trying to use Window Blinds' SkinStudio to create a simple Dark Mode Theme for Windows that won't change the interface, but just recolor these older apps like Datacad to match Windows 10 native dark mode. If I get it figured out I will share it here on the forum, but since Classic Color Panel works well enough it's not high on my priority list. EDIT: Actually they just released a new version of the Classic Color Panel app on the 11th of this month and it's profile loading is working great now so you can color individual programs instead of all programs. I now have Datacad set to load it's own profile which is awesome! and you no longer have to log out and back in to tweak.