[IMP] runbot_merge: tune dark mode colors

- Make the body color slightly darker for better contrast (makes it
  lighter when inverted).
- Make backgrounds lighter in dark mode.
- Hard-code success as well as danger and update the latter to make it
  lighter.

  Ideally this should be computed in oklch but sadly libsass doesn't
  support oklch computations, and there is no way to extract
  individual channels in CSS so there's no way to generate the `-rgb`
  variables.
- Hard-code the dark mode background to a dark gray instead of a black
  (using light-gray in light mode looks weird so keep white).
- Fix text and link color in alerts, I clearly still don't understand
  how CSS variables work and I don't care anymore, fuck'em.

Part of #1088

runbot_merge/static/scss/primary_variables.scss

@@ -15,13 +15,11 @@ $theme-colors: (
   "info":       $info,
   "warning":    $warning,
   "danger":     $danger,
-  // gray-100
   "light":      $light,
-  // gray-900
   "dark":       $dark,
 );
-// gray-600
-$body-color: #6c757d;
+// gray-700
+$body-color: #495057;
 // same as table-bg and alert-bg
 $bg-scale: -80%;
 

runbot_merge/static/scss/revariable.scss

@@ -27,7 +27,7 @@
     $ibc: invlight($body-color);
     --body-color: #{$ibc};
     --body-color-rgb: #{to-rgb($ibc)};
-    $ibg: invlight($body-bg);
+    $ibg: $gray-900;
     --body-bg: #{$ibg};
     --body-bg-rgb: #{to-rgb($ibg)};
 
@@ -39,10 +39,11 @@
       --#{$color}-bg-rgb: #{to-rgb(hsl(
         hue($value),
         min(saturation($value) + 16%, 100%),
-        10%,
+        15%,
       ))};
     }
-    --danger-bg-rgb: 41, 10, 10;
+    --success-bg-rgb: 17, 68, 28;
+    --danger-bg-rgb: 96, 32, 32;
 }
 
 :root.dark {
@@ -85,11 +86,18 @@ h1, h2, h3, h4, h5, h6{
   .alert-#{$category} {
     // inline alert-variant as it can't handle ~~the truth~~ CSS variables
     background-color: rgba(var(--#{$category}-bg-rgb), var(--bg-opacity));
-    border-color: color-mix(in rgb, var(--#{$category}-rgb) #{$alert-border-scale}, var(--light));
-    --alert-text: color-mix(in rgb, var(--#{$category}-rgb) #{$alert-color-scale}, var(--dark));
-    color: var(--alert-text);
-    .alert-link {
-      color: color-mix(in rgb, var(--alert-text) 20%, var(--dark));
+    border-color: color-mix(in srgb, var(--#{$category}) #{-$alert-border-scale}, var(--light));
+    color: color-mix(in srgb, var(--#{$category}) #{$alert-color-scale}, var(--dark));
+    a {
+      color: color-mix(
+        in srgb,
+        color-mix(
+          in srgb,
+          var(--#{$category}) #{$alert-color-scale},
+          var(--dark)
+        ) 80%,
+        var(--dark)
+      );
     }
   }
   .table-#{$category} {