Fix #308421: Use terminal theme background for composition view instead of hardcoded black

- Replace hardcoded black background (#000) in .xterm .composition-view with CSS variable
- Now uses var(--vscode-terminal-background) to respect terminal color theme
- Fallback to panel background, then hardcoded black if variables unavailable
- Fixes black background appearing after space character in terminal
- Affects IME composition view and applies consistently across all terminal themes
- Resolves issue on Windows PowerShell and other shells with IME input

Author: rupeshkumar-555

src/vs/workbench/contrib/terminal/browser/media/xterm.css

@@ -81,8 +81,8 @@
 }
 
 .xterm .composition-view {
-	/* TODO: Composition position got messed up somewhere */
-	background: #000;
+	/* Use terminal theme background instead of hardcoded black to prevent black background from showing after spaces */
+	background: var(--vscode-terminal-background, var(--vscode-panel-background, #000));
 	color: #FFF;
 	display: none;
 	position: absolute;