Improve styling.

src/palette-dark.css

@@ -0,0 +1,47 @@
+/* Dark palette: the colour half of the stylesheet.
+ *
+ * `appearance.zig` prepends exactly one of `palette-dark.css` or
+ * `palette-light.css` to `style.css` and loads the pair as a single provider,
+ * so every named colour below has a counterpart in the other file. Adding one
+ * here means adding it there too — a name defined in only one palette is a
+ * rule that silently stops applying in the other scheme.
+ *
+ * Deep navy rather than neutral grey, lifted a little at each step so the
+ * three surfaces (window, sidebar, pane) separate without any of them
+ * reading as light. `pp_surface` is duplicated in `theme.zig` as the terminal
+ * renderer's default background; the two have to stay equal or the frame
+ * around a terminal stops matching its contents. */
+
+/* Surfaces, darkest first. The window colour shows through as the gutter
+   between panes, so it sits below the panes rather than beside them. */
+@define-color pp_bg                    #070c15;
+@define-color pp_sidebar               #0b1320;
+@define-color pp_surface               #0e1624;
+@define-color pp_surface_raised        #131f30;
+@define-color pp_surface_raised_active #182740;
+
+@define-color pp_border                #1d2b3f;
+@define-color pp_border_strong         #2a3c55;
+
+/* Text, brightest first. `faint` is for things that are labelled rather than
+   read — pane titles of panes you aren't in, dialog hints. */
+@define-color pp_text                  #dde6f4;
+@define-color pp_text_dim              #9aabc2;
+@define-color pp_text_faint            #6a7c93;
+
+@define-color pp_row_hover             #162233;
+@define-color pp_row_selected          #1c2c45;
+
+/* Signal's ultramarine. `strong` is the variant that has to stay legible as a
+   small mark on a surface — icons, dots, the state bar down a row's edge —
+   which is why it gets brighter here and darker in the light palette. */
+@define-color pp_accent                #3a76f0;
+@define-color pp_accent_strong         #6191f3;
+@define-color pp_accent_muted          #2f5296;
+
+/* The three states that are news. Kept away from the accent on purpose: the
+   accent means "here" and these mean "something happened", and a sidebar
+   where those are the same colour answers neither question. */
+@define-color pp_ok                    #3ecf8e;
+@define-color pp_warn                  #f0b849;
+@define-color pp_err                   #f2717b;