Additional settings / "Visual" tab ([Setup] menu)

Window Opacity / Active
Window Opacity / Inactive
You can set up the value of opacity when the window is active (on focus) and not active (non-focus). The range of opacity is from 0 to 255. The more reduces the this value, the more window transparency is blinding.
See Background transparency.
MouseCursor
Mouse cursor can have one of the 4 shapes listed below:
ARROW = standard arrow cursor
IBEAM = ibeam cursor(default)
CROSS = cross cursor
HAND  = hand cursor
Font quality
Specifies the font quality. This feature will work well only if the operating system supports.
Default         = Dependent on OS configuration(Smooth edges of screen fonts).
Non-Antialiased = Disable the anti-aliasing.
Antialiased     = Enable the anti-aliasing.
ClearType       = Use the ClearType.
ANSI color
Specifies the RGB value of the color for PC style and aixterm style 16 colors mode.
Enable Bold attribute color
Enable Bold attribute font
Enable Underline attribute color
Enable Underline attribute font
Enable Blink attribute color
Enable Reverse attribute color
Enable URL attribute color
Enable URL attribute font
Enables each color attributes and color setting.
Enable ANSI color
If ANSI color is disabled, 16 colors and 256 colors modes are also disabled.
FlickerLessMove
Reduces flicker of drawing in a window when window is moved or resized.
Implementation:
Set SWP_NOCOPYBITS in flags of the WINDOWPOS structure (lParam) in WM_WINDOWPOSCHANGING message.

Theme

Looks of Tera Term can be changed.
It is not a Windows theme (dark theme, etc.) setting.

Theme Editor
Open Theme editor.
FastSizeMove
Faster drawing by omitting drawing of the window background (BG theme) during window resizing, moving, and Z-order changes.
This can be used when using the BG theme.
startup
Sets the theme to be used at startup.
no use
No theme is used
Fixed theme file
Loads a specific theme file
Random Theme file
Randomly load themes from theme files folder
Susie Plug-in path
Path where plugin file is stored.
Susie Plug-in is an external file to load various image files using mage viewer software Susie.
Following information was referenced.