Styling option for separating terminal view background from background color (#17611)

Closes #17313

Release Notes:

- Added theme styling option to separate terminal background view from
terminal background color, for transparent terminal backgrounds

crates/theme/src/styles/colors.rs

@@ -175,7 +175,7 @@ pub struct ThemeColors {
     // ===
     // Terminal
     // ===
-    /// Terminal background color.
+    /// Terminal layout background color.
     pub terminal_background: Hsla,
     /// Terminal foreground color.
     pub terminal_foreground: Hsla,
@@ -184,6 +184,9 @@ pub struct ThemeColors {
     /// Dim terminal foreground color.
     pub terminal_dim_foreground: Hsla,
 
+    /// Terminal ansi background color.
+    pub terminal_ansi_background: Hsla,
+
     /// Black ANSI terminal color.
     pub terminal_ansi_black: Hsla,
     /// Bright black ANSI terminal color.