fix(theme): dark theme + landing toggle now actually applies

Two bugs:
1. [data-theme="dark"] block was defined BEFORE :root in source CSS,
   so :root cascade overrode it. Moved to AFTER :root and bumped
   specificity to html[data-theme="dark"] to beat plain :root.
2. Theme toggle button only existed in the topbar, which is hidden
   on landing/login/sso-callback scenes. Added a ThemeToggle to
   the Landing header so users can flip theme from the home page.

Confidence: high
Scope-risk: narrow

qa/probe_dom.mjs

@@ -0,0 +1,23 @@
+import { chromium } from "playwright";
+const b = await chromium.launch({
+  headless: true,
+  args: ["--host-resolver-rules=MAP canvas.flow-master.ai 65.21.71.186", "--ignore-certificate-errors"],
+});
+const ctx = await b.newContext({ viewport: { width: 1440, height: 900 } });
+const p = await ctx.newPage();
+await p.goto("https://canvas.flow-master.ai/", { waitUntil: "networkidle" });
+await p.waitForTimeout(1500);
+await p.locator("input[type='email']").fill("dev@flow-master.ai");
+const devBtn = p.locator("button", { hasText: /developer|dev-login/i });
+if (await devBtn.count() > 0) await devBtn.click();
+await p.waitForTimeout(3000);
+
+console.log("=== ALL .link-btn buttons ===");
+const buttons = await p.$$eval(".link-btn", (els) => els.map((e) => ({
+  class: e.className,
+  text: (e.innerText || "").slice(0, 50),
+  aria: e.getAttribute("aria-label"),
+  title: e.getAttribute("title"),
+})));
+buttons.forEach(b => console.log(JSON.stringify(b)));
+await b.close();

qa/probe_theme.mjs

@@ -0,0 +1,29 @@
+import { chromium } from "playwright";
+const b = await chromium.launch({
+  headless: true,
+  args: ["--host-resolver-rules=MAP canvas.flow-master.ai 65.21.71.186", "--ignore-certificate-errors"],
+});
+const ctx = await b.newContext({ viewport: { width: 1440, height: 900 } });
+const p = await ctx.newPage();
+await p.goto("https://canvas.flow-master.ai/", { waitUntil: "networkidle" });
+await p.waitForTimeout(2000);
+
+// Sign in first
+await p.locator("input[type='email']").fill("dev@flow-master.ai");
+const devBtn = p.locator("button", { hasText: /developer|dev-login/i });
+if (await devBtn.count() > 0) await devBtn.click();
+await p.waitForTimeout(2500);
+
+console.log("after login theme:", await p.evaluate(() => document.documentElement.dataset.theme));
+console.log("after login body bg:", await p.evaluate(() => getComputedStyle(document.body).backgroundColor));
+
+const toggle = p.locator(".theme-toggle, .link-btn.theme-toggle, button[aria-label*='theme']").first();
+const count = await toggle.count();
+console.log("toggle count:", count);
+if (count > 0) {
+  await toggle.click();
+  await p.waitForTimeout(800);
+  console.log("after click theme:", await p.evaluate(() => document.documentElement.dataset.theme));
+  console.log("after click body bg:", await p.evaluate(() => getComputedStyle(document.body).backgroundColor));
+}
+await b.close();

qa/probe_theme2.mjs

@@ -0,0 +1,28 @@
+import { chromium } from "playwright";
+const b = await chromium.launch({
+  headless: true,
+  args: ["--host-resolver-rules=MAP canvas.flow-master.ai 65.21.71.186", "--ignore-certificate-errors"],
+});
+const ctx = await b.newContext({ viewport: { width: 1440, height: 900 } });
+const p = await ctx.newPage();
+await p.goto("https://canvas.flow-master.ai/", { waitUntil: "networkidle" });
+await p.waitForTimeout(1500);
+// Get the computed --bp-paper at root
+console.log("html dataset.theme:", await p.evaluate(() => document.documentElement.dataset.theme));
+console.log("--bp-paper (root):", await p.evaluate(() => getComputedStyle(document.documentElement).getPropertyValue("--bp-paper")));
+console.log("body background:", await p.evaluate(() => getComputedStyle(document.body).backgroundColor));
+
+// Force toggle data-theme via JS
+await p.evaluate(() => document.documentElement.dataset.theme = "dark");
+await p.waitForTimeout(200);
+console.log("after force dark:");
+console.log("  --bp-paper:", await p.evaluate(() => getComputedStyle(document.documentElement).getPropertyValue("--bp-paper")));
+console.log("  body bg:", await p.evaluate(() => getComputedStyle(document.body).backgroundColor));
+
+await p.evaluate(() => document.documentElement.dataset.theme = "light");
+await p.waitForTimeout(200);
+console.log("after force light:");
+console.log("  --bp-paper:", await p.evaluate(() => getComputedStyle(document.documentElement).getPropertyValue("--bp-paper")));
+console.log("  body bg:", await p.evaluate(() => getComputedStyle(document.body).backgroundColor));
+
+await b.close();

qa/probe_theme3.mjs

@@ -0,0 +1,28 @@
+import { chromium } from "playwright";
+const b = await chromium.launch({
+  headless: true,
+  args: ["--host-resolver-rules=MAP canvas.flow-master.ai 65.21.71.186", "--ignore-certificate-errors"],
+});
+const ctx = await b.newContext({ viewport: { width: 1440, height: 900 } });
+const p = await ctx.newPage();
+await p.goto("https://canvas.flow-master.ai/", { waitUntil: "domcontentloaded" });
+await p.waitForLoadState("networkidle");
+await p.waitForTimeout(2000);
+
+// Read the data-theme attribute repeatedly
+console.log("docElement.dataset.theme:", await p.evaluate(() => document.documentElement.dataset.theme));
+console.log("docElement attributes:", await p.evaluate(() => document.documentElement.getAttributeNames().join(",")));
+console.log("html outerHTML head:", await p.evaluate(() => document.documentElement.outerHTML.slice(0, 200)));
+console.log("computed --bp-paper:", await p.evaluate(() => getComputedStyle(document.documentElement).getPropertyValue("--bp-paper")));
+
+// Try setting dark via JS and reading
+await p.evaluate(() => {
+  document.documentElement.setAttribute("data-theme", "dark");
+});
+await p.waitForTimeout(500);
+console.log("\nafter setAttribute('data-theme','dark'):");
+console.log("dataset.theme:", await p.evaluate(() => document.documentElement.dataset.theme));
+console.log("--bp-paper:", await p.evaluate(() => getComputedStyle(document.documentElement).getPropertyValue("--bp-paper")));
+console.log("body bg:", await p.evaluate(() => getComputedStyle(document.body).backgroundColor));
+
+await b.close();

qa/probe_topbar.mjs

@@ -0,0 +1,17 @@
+import { chromium } from "playwright";
+const b = await chromium.launch({
+  headless: true,
+  args: ["--host-resolver-rules=MAP canvas.flow-master.ai 65.21.71.186", "--ignore-certificate-errors"],
+});
+const ctx = await b.newContext({ viewport: { width: 1440, height: 900 } });
+const p = await ctx.newPage();
+await p.goto("https://canvas.flow-master.ai/", { waitUntil: "networkidle" });
+await p.waitForTimeout(1500);
+await p.locator("input[type='email']").fill("dev@flow-master.ai");
+const devBtn = p.locator("button", { hasText: /developer|dev-login/i });
+if (await devBtn.count() > 0) await devBtn.click();
+await p.waitForTimeout(3000);
+console.log("topbar exists:", await p.locator(".topbar").count());
+console.log("topbar HTML:");
+console.log(await p.evaluate(() => document.querySelector(".topbar")?.outerHTML?.slice(0, 2000)));
+await b.close();