[DevTools] Make Toggle hover state more visible (#32914)

This change adds a background color to Toggles to make them easier to
see. This is especially important when DevTools are not in focus, and
it's harder to see.

Test plan:
1. `yarn build:chrome:local`
2. Inspect components 
3. Hover over "Select an Element in page to inspect it"
4. Observe background change

packages/react-devtools-shared/src/devtools/constants.js

@@ -27,6 +27,7 @@ export const THEME_STYLES: {[style: Theme | DisplayDensity]: any, ...} = {
     '--color-background-selected': '#0088fa',
     '--color-button-background': '#ffffff',
     '--color-button-background-focus': '#ededed',
+    '--color-button-background-hover': 'rgba(0, 0, 0, 0.2)',
     '--color-button': '#5f6673',
     '--color-button-disabled': '#cfd1d5',
     '--color-button-active': '#0088fa',
@@ -174,6 +175,7 @@ export const THEME_STYLES: {[style: Theme | DisplayDensity]: any, ...} = {
     '--color-background-selected': '#178fb9',
     '--color-button-background': '#282c34',
     '--color-button-background-focus': '#3d424a',
+    '--color-button-background-hover': 'rgba(255, 255, 255, 0.2)',
     '--color-button': '#afb3b9',
     '--color-button-active': '#61dafb',
     '--color-button-disabled': '#4f5766',

packages/react-devtools-shared/src/devtools/views/Toggle.css

@@ -20,10 +20,17 @@
   background: var(--color-button-background);
   color: var(--color-button);
 }
+
 .ToggleOff:hover {
   color: var(--color-button-hover);
 }
 
+.ToggleOn:hover,
+.ToggleOff:hover {
+ background-color: var(--color-button-background-hover);
+}
+
+
 .ToggleOn,
 .ToggleOn:active {
   color: var(--color-button-active);