fix(chat): always use the lightest bg color we can find for the chat input box

plugins/amazonq/chat/jetbrains-community/src/software/aws/toolkits/jetbrains/services/amazonq/webview/theme/ThemeBrowserAdapter.kt

@@ -26,6 +26,7 @@
     }
 
     private fun buildJsCodeToUpdateTheme(theme: AmazonQTheme) = buildString {
+        val (bg, altBg, inputBg) = determineInputAndBgColor(theme)
         appendDarkMode(theme.darkMode)
 
         append("{\n")
@@ -42,14 +43,14 @@
         append(CssVariable.TextColorWeak, theme.inactiveText)
         append(CssVariable.TextColorDisabled, theme.inactiveText)
 
-        append(CssVariable.Background, theme.editorBackground)
-        append(CssVariable.BackgroundAlt, theme.background)
-        append(CssVariable.CardBackground, theme.editorBackground)
-        append(CssVariable.CardBackgroundAlt, theme.background)
+        append(CssVariable.Background, bg)
+        append(CssVariable.BackgroundAlt, altBg)
+        append(CssVariable.CardBackground, bg)
+        append(CssVariable.CardBackgroundAlt, altBg)
         append(CssVariable.BorderDefault, theme.border)
         append(CssVariable.TabActive, theme.activeTab)
 
-        append(CssVariable.InputBackground, theme.textFieldBackground)
+        append(CssVariable.InputBackground, inputBg)
 
         append(CssVariable.ButtonBackground, theme.buttonBackground)
         append(CssVariable.ButtonForeground, theme.buttonForeground)
@@ -110,4 +111,15 @@
 
     // Some font names have characters that require them to be wrapped in quotes in the CSS variable, for example if they have spaces or a period.
     private fun Font.toCssFontFamily(fallback: String = "system-ui") = "\"$family\", $fallback"
+
+    // darkest = bg, second darkest is alt bg, lightest is input bg
+    private fun determineInputAndBgColor(theme: AmazonQTheme): Triple<Color, Color, Color> {
+        val colors = arrayOf(theme.editorBackground, theme.background, theme.textFieldBackground).sortedWith(
+            Comparator.comparing {
+                // luma calculation for brightness
+                (0.2126 * it.red) + (0.7152 * it.green) + (0.0722 * it.blue)
+            }
+        )
+        return Triple(colors[0], colors[1], colors[2])
+    }
 }