feat(ui): tweak link colors

- make the `SDXLConcatLink` icon match existing colors in light mode
- make the link toggle button accent color when active (its not super obvious but at least there is *some* visual difference for the button)

invokeai/frontend/web/src/features/sdxl/components/ParamSDXLConcatButton.tsx

@@ -33,7 +33,7 @@ export default function ParamSDXLConcatButton() {
         insetInlineEnd: 1,
         top: shouldPinParametersPanel ? 12 : 20,
         border: 'none',
-        color: 'base.500',
+        color: shouldConcatSDXLStylePrompt ? 'accent.500' : 'base.500',
         _hover: {
           bg: 'none',
         },

invokeai/frontend/web/src/features/sdxl/components/SDXLConcatLink.tsx

@@ -72,7 +72,8 @@ export default function SDXLConcatLink() {
           top: '3px',
           p: 1,
           borderRadius: 4,
-          bg: 'accent.200',
+          bg: 'accent.400',
+          color: 'base.50',
           _dark: {
             bg: 'accent.500',
           },