restore original Doxygen contrasts

the contrast was decreased at
https://github.com/deltachat/deltachat-core-rust/pull/4136 ,
there were also suggestions to fix it there,
but it was probably just forgotten :)

this pr increases the contrast
between code background and code font again to the level it was before
and also to what similar themes are doing.

deltachat-ffi/Doxyfile.css

@@ -1,11 +1,11 @@
 
 :root {
-	--accent: hsl(0 0% 70%);
+	--accent: hsl(0 0% 85%);
 }
 
 @media (prefers-color-scheme: dark) {
 	:root {
-		--accent: hsl(0 0% 30%);
+		--accent: hsl(0 0% 25%);
 	}
 }