update panel input border logic (microsoft#165966)

fixes microsoft#165843

Author: sbatten

src/vs/workbench/browser/parts/panel/media/panelpart.css

@@ -47,7 +47,7 @@
 }
 
 .monaco-workbench .part.panel .monaco-inputbox {
-	border-color: var(--vscode-panelInput-border) !important;
+	border-color: var(--vscode-panelInput-border, transparent) !important;
 }
 
 .monaco-workbench .part.basepanel > .title > .panel-switcher-container > .monaco-action-bar .action-item:focus .active-item-indicator:before {

src/vs/workbench/common/theme.ts

@@ -4,7 +4,7 @@
  *--------------------------------------------------------------------------------------------*/
 
 import { localize } from 'vs/nls';
-import { registerColor, editorBackground, contrastBorder, transparent, editorWidgetBackground, textLinkForeground, lighten, darken, focusBorder, activeContrastBorder, editorWidgetForeground, editorErrorForeground, editorWarningForeground, editorInfoForeground, treeIndentGuidesStroke, errorForeground, listActiveSelectionBackground, listActiveSelectionForeground, editorForeground, toolbarHoverBackground } from 'vs/platform/theme/common/colorRegistry';
+import { registerColor, editorBackground, contrastBorder, transparent, editorWidgetBackground, textLinkForeground, lighten, darken, focusBorder, activeContrastBorder, editorWidgetForeground, editorErrorForeground, editorWarningForeground, editorInfoForeground, treeIndentGuidesStroke, errorForeground, listActiveSelectionBackground, listActiveSelectionForeground, editorForeground, toolbarHoverBackground, inputBorder } from 'vs/platform/theme/common/colorRegistry';
 import { IColorTheme } from 'vs/platform/theme/common/themeService';
 import { Color } from 'vs/base/common/color';
 import { ColorScheme } from 'vs/platform/theme/common/theme';
@@ -352,10 +352,10 @@ export const PANEL_ACTIVE_TITLE_BORDER = registerColor('panelTitle.activeBorder'
 }, localize('panelActiveTitleBorder', "Border color for the active panel title. Panels are shown below the editor area and contain views like output and integrated terminal."));
 
 export const PANEL_INPUT_BORDER = registerColor('panelInput.border', {
-	dark: null,
+	dark: inputBorder,
 	light: Color.fromHex('#ddd'),
-	hcDark: null,
-	hcLight: null
+	hcDark: inputBorder,
+	hcLight: inputBorder
 }, localize('panelInputBorder', "Input box border for inputs in the panel."));
 
 export const PANEL_DRAG_AND_DROP_BORDER = registerColor('panel.dropBorder', {