fix: the page draws its text black, as a word processor does

The sheet took the app's slate (#1e293b), so every uncoloured run
differed from LibreOffice — and from a run the same document colours
#000000 explicitly. Light and dark themes now paint the page black on
white; allBlack still inverts it, and the PDF export no longer has to
override anything.

src/lib/storage/CLAUDE.md

@@ -32,4 +32,4 @@ Both margins and orientation are passed into the ODT export so the exported docu
 
 ## Themes (`theme.ts`, `styles/global.css`)
 
-Four modes: `light`, `dark`, `allBlack` (forces font colors white), `auto` (follows `prefers-color-scheme`). Applied by setting `data-theme` on `<html>`; CSS variables for each theme live in `global.css`.
+Four modes: `light`, `dark`, `allBlack` (forces font colors white), `auto` (follows `prefers-color-scheme`). Applied by setting `data-theme` on `<html>`; CSS variables for each theme live in `global.css`. The sheet keeps its own pair through light and dark — white paper, **black** text, the colour LibreOffice and Word draw an uncoloured run in (the app's slate would also differ from a run the file colours `#000000`); only `allBlack` inverts it.

src/styles/global.css

@@ -302,7 +302,9 @@
   --color-toolbar-bg: #ffffff;
   --color-btn-hover: #cbd5e1;
   --color-page-bg: #ffffff;
-  --color-page-text: #1e293b;
+  /* The sheet is the document, so its text is the black LibreOffice and Word draw —
+     the app's own slate would differ from a run the file colours #000000. */
+  --color-page-text: #000000;
   --color-page-border: #e2e8f0;
   --scrollbar-thumb: #cbd5e1;
   }
@@ -317,7 +319,7 @@
     --color-toolbar-bg: #161b22;
     --color-btn-hover: #3b4350;
     --color-page-bg: #ffffff;
-    --color-page-text: #1e293b;
+    --color-page-text: #000000;
     --color-page-border: #e2e8f0;
     /* Primary (active-button color) so the toolbar scrollbar stands out on dark. */
     --scrollbar-thumb: var(--color-primary);