diff --git a/apps/server/src/environmentTheme.test.ts b/apps/server/src/environmentTheme.test.ts
index 1e8b538a665ae..241c89546c228 100644
--- a/apps/server/src/environmentTheme.test.ts
+++ b/apps/server/src/environmentTheme.test.ts
@@ -2,6 +2,7 @@ import {
   DEFAULT_SERVER_SETTINGS,
   EnvironmentThemeFile,
   ServerSettingsError,
+  THEME_CSS_MAX_LENGTH,
 } from "@t3tools/contracts";
 import * as NodeServices from "@effect/platform-node/NodeServices";
 import { assert, it } from "@effect/vitest";
@@ -20,6 +21,12 @@ import { ServerSettingsService } from "./serverSettings.ts";
 import { symlinksSupported } from "@t3tools/shared/testing/symlinks";
 
 const encodeThemeFile = Schema.encodeSync(Schema.fromJsonString(EnvironmentThemeFile));
+/** Writes non-ASCII characters as `\uXXXX` escapes, as encoders such as Python's json.dumps do. */
+const encodeThemeFileEscaped = (theme: EnvironmentThemeFile) =>
+  encodeThemeFile(theme).replace(
+    /[\u0080-\uffff]/g,
+    (char) => `\\u${char.charCodeAt(0).toString(16).padStart(4, "0")}`,
+  );
 
 const NIGHTFALL_THEME: EnvironmentThemeFile = {
   name: "Nightfall",
@@ -190,7 +197,10 @@ it.layer(NodeServices.layer)("environment theme", (it) => {
   );
 
   it.effect("does not replace a theme file it could not put back", () => {
-    const oversized = JSON.stringify({ ...NIGHTFALL_THEME, padding: "x".repeat(40_000) });
+    const oversized = JSON.stringify({
+      ...NIGHTFALL_THEME,
+      padding: "x".repeat(EnvironmentTheme.MAX_THEME_FILE_BYTES),
+    });
     return withEnvironmentThemes(
       { [`${REFUSED_THEME_ID}.json`]: oversized },
       Effect.gen(function* () {
@@ -298,14 +308,14 @@ it.layer(NodeServices.layer)("environment theme", (it) => {
     const large = (name: string): EnvironmentThemeFile => ({
       ...NIGHTFALL_THEME,
       name,
-      css: `/* ${"x".repeat(19_900)} */`,
+      css: `/* ${"界".repeat(THEME_CSS_MAX_LENGTH - 6)} */`,
     });
     return withEnvironmentThemes(
       {
         ...Object.fromEntries(
-          Array.from({ length: 9 }, (_, index) => [
+          Array.from({ length: 6 }, (_, index) => [
             `aa-${index}.json`,
-            encodeThemeFile(large(`Large ${index}`)),
+            encodeThemeFileEscaped(large(`Large ${index}`)),
           ]),
         ),
         "zz-look.json": encodeThemeFile(NIGHTFALL_THEME),
@@ -341,7 +351,10 @@ it.layer(NodeServices.layer)("environment theme", (it) => {
     withEnvironmentThemes(
       {
         ...Object.fromEntries(
-          Array.from({ length: 7 }, (_, index) => [`junk-${index}.json`, "{".repeat(30_000)]),
+          Array.from({ length: 7 }, (_, index) => [
+            `junk-${index}.json`,
+            "{".repeat(EnvironmentTheme.MAX_THEME_FILE_BYTES),
+          ]),
         ),
         "zz-valid.json": encodeThemeFile(NIGHTFALL_THEME),
       },
diff --git a/apps/server/src/environmentTheme.ts b/apps/server/src/environmentTheme.ts
index b658870f2e960..1e7f4d3052301 100644
--- a/apps/server/src/environmentTheme.ts
+++ b/apps/server/src/environmentTheme.ts
@@ -9,6 +9,7 @@ import {
   EnvironmentThemeFile,
   EnvironmentThemeId,
   environmentThemeFileHasColors,
+  THEME_CSS_MAX_LENGTH,
 } from "@t3tools/contracts";
 import { fromJsonStringPretty } from "@t3tools/shared/schemaJson";
 import { themeCssLoadsRemoteFiles } from "@t3tools/shared/themeCss";
@@ -44,9 +45,12 @@ const isEnvironmentThemeId = Schema.is(EnvironmentThemeId);
 const THEME_FILE_SUFFIX = ".json";
 
 const MAX_THEME_FILES = 32;
-/** Shared with publishing so it cannot accept files the watcher skips. */
-export const MAX_THEME_FILE_BYTES = 32 * 1024;
-const MAX_THEME_TOTAL_BYTES = 192 * 1024;
+/**
+ * Shared with publishing so it cannot accept files the watcher skips. Fits the longest CSS in any
+ * JSON encoding, at up to 6 bytes per character for a `\uXXXX` escape, plus the colors.
+ */
+export const MAX_THEME_FILE_BYTES = 6 * THEME_CSS_MAX_LENGTH + 16 * 1024;
+const MAX_THEME_TOTAL_BYTES = 6 * MAX_THEME_FILE_BYTES;
 
 interface PublishedThemes {
   readonly seq: number;
diff --git a/packages/contracts/src/server.ts b/packages/contracts/src/server.ts
index 88fa0220fe920..e224471c19607 100644
--- a/packages/contracts/src/server.ts
+++ b/packages/contracts/src/server.ts
@@ -622,7 +622,7 @@ const EnvironmentThemeColors = Schema.Record(
   TrimmedNonEmptyString.check(Schema.isMaxLength(64)),
 );
 
-export const THEME_CSS_MAX_LENGTH = 20_000;
+export const THEME_CSS_MAX_LENGTH = 50_000;
 
 const environmentThemeFields = {
   version: Schema.optional(Schema.Literal(1)),
diff --git a/packages/shared/src/themeCss.ts b/packages/shared/src/themeCss.ts
index 48b1259d0377b..ee933f48a171e 100644
--- a/packages/shared/src/themeCss.ts
+++ b/packages/shared/src/themeCss.ts
@@ -67,7 +67,7 @@ export const THEME_GUIDE = [
   "",
   "CSS: `css` applies after the app's styles. Use `.dark` or `:root:not(.dark)` for one appearance.",
   "It cannot load files: no @import, no image-set(), and url() only with data: or #id.",
-  "A background image must be embedded as a data: address. `css` holds 20,000 characters, so keep the image near 10 KB.",
+  "A background image must be embedded as a data: address. `css` holds 50,000 characters, so keep the image near 30 KB.",
   "The app's own rules can be more specific. If a rule has no effect, add !important.",
   "It cannot reach the terminal text, the file tree, or diff internals. Use color roles for those.",
   "Native scrollbars take `::-webkit-scrollbar` and `::-webkit-scrollbar-thumb`.",