Bug 1555570 - Slightly darken background colors in devtools dark theme; r=rcaliman

Following the color tests in https://bugzilla.mozilla.org/show_bug.cgi?id=1555570#c7
- Use a slightly darker background color for the body background, to improve text-to-background contrast
- As a result, tweak the sidebar and toolbar background, and the selection hover background

Differential Revision: https://phabricator.services.mozilla.com/D35154

--HG--
extra : moz-landing-system : lando

devtools/client/themes/variables.css

@@ -116,19 +116,19 @@
 }
 
 :root.theme-dark {
-  --theme-body-background: var(--grey-80);
+  --theme-body-background: #232327;
-  --theme-sidebar-background: var(--grey-85);
+  --theme-sidebar-background: #18181a;
   --theme-contrast-background: #ffb35b;
 
   /* Toolbar */
   --theme-tab-toolbar-background: var(--grey-90);
-  --theme-toolbar-background: var(--grey-85);
+  --theme-toolbar-background: #18181a;
   --theme-toolbar-color: var(--grey-40);
   --theme-toolbar-selected-color: white;
   --theme-toolbar-highlighted-color: var(--green-50);
-  --theme-toolbar-background-hover: #20232B;
+  --theme-toolbar-background-hover: #232327;
   --theme-toolbar-background-alt: var(--grey-85);
-  --theme-toolbar-hover: #252526;
+  --theme-toolbar-hover: #232327;
   --theme-toolbar-hover-active: #252526;
 
   /* Buttons */
@@ -137,13 +137,13 @@
 
   /* Selection */
   --theme-selection-background: #204E8A;
-  --theme-selection-background-hover: #353B48;
+  --theme-selection-background-hover: #303844;
   --theme-selection-focus-background: var(--grey-60);
   --theme-selection-focus-color: var(--grey-30);
   --theme-selection-color: #ffffff;
 
   /* Border color that splits the toolbars/panels/headers. */
-  --theme-splitter-color: #3c3c3d;
+  --theme-splitter-color: var(--grey-70);
   --theme-emphasized-splitter-color: var(--grey-60);
   --theme-emphasized-splitter-color-hover: var(--grey-50);