Fix terminal's selection rendering on top

Fixes #29040

src/vs/workbench/parts/terminal/electron-browser/media/xterm.css

@@ -136,6 +136,9 @@
 	position: absolute;
 	left: 0;
 	bottom: 0;
+	z-index: 1;
+	opacity: 0.3;
+	pointer-events: none;
 }
 
 .monaco-workbench .panel.integrated-terminal .xterm .xterm-selection div {

src/vs/workbench/parts/terminal/electron-browser/terminalColorRegistry.ts

@@ -20,9 +20,9 @@ export const TERMINAL_FOREGROUND_COLOR = registerColor('terminal.foreground', {
 	hc: '#FFFFFF'
 }, nls.localize('terminal.foreground', 'The foreground color of the terminal.'));
 export const TERMINAL_SELECTION_BACKGROUND_COLOR = registerColor('terminal.selectionBackground', {
-	light: '#B0B0B0',
-	dark: '#404040',
-	hc: '#808080'
+	light: '#000',
+	dark: '#FFF',
+	hc: '#FFF'
 }, nls.localize('terminal.selectionBackground', 'The selection background color of the terminal.'));
 
 const ansiColorMap = {