report: improve devtools dark mode rendering

[paulirish]

fixes #3827 (what wasn't fixed in #4190)

Before and after:

FWIW, This does require some DevTools-side changes as well:

diff --git a/third_party/WebKit/Source/devtools/front_end/audits2/audits2Panel.css b/third_party/WebKit/Source/devtools/front_end/audits2/audits2Panel.css
index 3e4d2e86f0bb..200a48388d91 100644
--- a/third_party/WebKit/Source/devtools/front_end/audits2/audits2Panel.css
+++ b/third_party/WebKit/Source/devtools/front_end/audits2/audits2Panel.css
@@ -49,7 +49,19 @@
 .lh-root {
   --report-menu-width: 0;
   user-select: initial;
-  background: #fff;
+  --lh-bg-color: #fff;
+  background-color: var(--lh-bg-color);
+}
+
+.-theme-with-dark-background .lh-root {
+  --lh-bg-color: hsl(0, 0%, 14%);
+  --secondary-text-color: hsl(0, 0%, 66%);
+  --report-secondary-border-color: hsl(0, 0%, 8%);
+}
+
+.-theme-with-dark-background .lh-root .lh-gauge {
+  --circle-background: hsl(0, 0%, 27%);
+  --inset-color: var(--lh-bg-color);
 }
 
 .lh-root .lh-container {

[patrickhulce]

much better 👍