There are two major Theme options.
Use Edit menu > Application-Settings > General tab > theme Options : Target Theme to load the theme.
Charcoal Dark Slate : this theme uses Dark Borders, menus and dialogs with contrasting toolbars icons.
Windows10 : this theme uses Light Borders, menus and dialogs, with contrasting colours for the icons.
You cannot edit the theme.
Each Theme has a recommended Style.
Use the Load button [at the bottom of the Application-Settings dialog] to load the Dark.xml or Light.xml as recommended above.
Dark Style is recommended for the Charcoal Dark Slate theme
Light Styles recommended for the Windows10 theme
In addition to the Theme and Style, you can edit other settings: e.g.: icon size, text size, element colours, number format, ...
These settings, and the Theme, Style, are saved to MechDesigner.ini* file when you exit MechDesigner to become the new default style.
The MechDesigner.ini file is loaded again when you start MechDesigner, to become the active style.
The MechDesigner.ini file loads your preferred settings each time you start MechDesigner.
You may not need to make any other changes when you are the only designer on the PC. However, different designers may prefer a different style on the same PC.
Each user should save their settings as a named-style.xml with the SAVE button.
Use the LOAD button to load named-style.xml, to re-apply their preferences and settings.
Copy and distributed the named-style.xml file to other PCs.
Rename named-style.xml to Company-Style.xml , to become a company-style, for example.
Each user must use the Load button to make this the active style on the PC, of course.
* See Files on your PC
Themes, Style and Settings: