diff --git a/.changeset/serious-pens-grab.md b/.changeset/serious-pens-grab.md
new file mode 100644
index 0000000..56c6599
--- /dev/null
+++ b/.changeset/serious-pens-grab.md
@@ -0,0 +1,13 @@
+---
+'@vivliostyle/theme-gutenberg': patch
+'@vivliostyle/theme-academic': patch
+'@vivliostyle/theme-techbook': patch
+'@vivliostyle/theme-epub3j': patch
+'@vivliostyle/theme-bunko': patch
+'@vivliostyle/theme-slide': patch
+'create-vivliostyle-theme': patch
+'@vivliostyle/theme-base': patch
+---
+
+Add a `main` property for package.json.
+This is useful for use with third-party libraries that are expected to have a `main` field in package.json.
diff --git a/packages/@vivliostyle/theme-academic/package.json b/packages/@vivliostyle/theme-academic/package.json
index ff1a83c..60117f2 100644
--- a/packages/@vivliostyle/theme-academic/package.json
+++ b/packages/@vivliostyle/theme-academic/package.json
@@ -3,6 +3,7 @@
   "description": "Academic theme",
   "version": "1.0.0",
   "author": "Vivliostyle <mail@vivliostyle.org>",
+  "main": "theme.css",
   "scripts": {
     "build": "vivliostyle build",
     "preview": "vivliostyle preview",
@@ -46,7 +47,7 @@
     "theme": {
       "name": "Academic",
       "category": "academic",
-      "style": "./theme.css",
+      "style": "theme.css",
       "topics": [
         "Academic",
         "Report"
diff --git a/packages/@vivliostyle/theme-base/package.json b/packages/@vivliostyle/theme-base/package.json
index 545b68d..43ba517 100644
--- a/packages/@vivliostyle/theme-base/package.json
+++ b/packages/@vivliostyle/theme-base/package.json
@@ -3,6 +3,7 @@
   "description": "Base theme and CSS toolkit for Vivliostyle themes",
   "version": "1.0.0",
   "author": "Vivliostyle <mail@vivliostyle.org>",
+  "main": "theme-all.css",
   "scripts": {
     "build": "vivliostyle build",
     "preview": "vivliostyle preview",
@@ -44,7 +45,7 @@
     "theme": {
       "name": "Vivliostyle Base Theme",
       "author": "Vivliostyle <mail@vivliostyle.org>",
-      "style": "./theme-all.css",
+      "style": "theme-all.css",
       "category": "misc",
       "topics": []
     }
diff --git a/packages/@vivliostyle/theme-bunko/package.json b/packages/@vivliostyle/theme-bunko/package.json
index a07bbf7..04fc768 100644
--- a/packages/@vivliostyle/theme-bunko/package.json
+++ b/packages/@vivliostyle/theme-bunko/package.json
@@ -3,6 +3,7 @@
   "description": "文庫用のテーマ",
   "version": "1.0.0",
   "author": "Vivliostyle <mail@vivliostyle.org>",
+  "main": "theme.css",
   "scripts": {
     "build": "vivliostyle build",
     "preview": "vivliostyle preview",
@@ -45,7 +46,7 @@
   "vivliostyle": {
     "theme": {
       "name": "Bunko",
-      "style": "./theme.css",
+      "style": "theme.css",
       "category": "novel",
       "topics": [
         "小説",
diff --git a/packages/@vivliostyle/theme-epub3j/package.json b/packages/@vivliostyle/theme-epub3j/package.json
index 388b797..a1d3575 100644
--- a/packages/@vivliostyle/theme-epub3j/package.json
+++ b/packages/@vivliostyle/theme-epub3j/package.json
@@ -3,6 +3,7 @@
   "description": "EPUB3 (Japanese) 日本語EPUB用テーマ（電書協EPUB3制作ガイド準拠）",
   "version": "1.0.0",
   "author": "Vivliostyle <mail@vivliostyle.org>",
+  "main": "theme.css",
   "scripts": {
     "build": "vivliostyle build",
     "preview": "vivliostyle preview",
@@ -43,7 +44,7 @@
   "vivliostyle": {
     "theme": {
       "name": "EPUB3 (Japanese)",
-      "style": "./theme.css",
+      "style": "theme.css",
       "category": "misc",
       "topics": []
     }
diff --git a/packages/@vivliostyle/theme-gutenberg/package.json b/packages/@vivliostyle/theme-gutenberg/package.json
index 3808cb1..fc2323e 100644
--- a/packages/@vivliostyle/theme-gutenberg/package.json
+++ b/packages/@vivliostyle/theme-gutenberg/package.json
@@ -3,6 +3,7 @@
   "description": "Book theme for latin font",
   "version": "1.0.0",
   "author": "Vivliostyle <mail@vivliostyle.org>",
+  "main": "theme.css",
   "scripts": {
     "build": "vivliostyle build",
     "preview": "vivliostyle preview",
@@ -38,7 +39,7 @@
     "theme": {
       "name": "Gutenberg",
       "author": "Vivliostyle <mail@vivliostyle.org>",
-      "style": "./theme.css",
+      "style": "theme.css",
       "category": "novel",
       "topics": []
     }
diff --git a/packages/@vivliostyle/theme-slide/package.json b/packages/@vivliostyle/theme-slide/package.json
index b096d56..0f5bcd3 100644
--- a/packages/@vivliostyle/theme-slide/package.json
+++ b/packages/@vivliostyle/theme-slide/package.json
@@ -3,6 +3,7 @@
   "description": "Slide theme",
   "version": "1.0.0",
   "author": "Vivliostyle <mail@vivliostyle.org>",
+  "main": "theme.css",
   "scripts": {
     "build": "vivliostyle build",
     "preview": "vivliostyle preview",
@@ -46,7 +47,7 @@
     "theme": {
       "name": "Slide",
       "category": "misc",
-      "style": "./theme.css",
+      "style": "theme.css",
       "topics": [
         "Slide"
       ]
diff --git a/packages/@vivliostyle/theme-techbook/package.json b/packages/@vivliostyle/theme-techbook/package.json
index 1bc7893..4ee4c95 100644
--- a/packages/@vivliostyle/theme-techbook/package.json
+++ b/packages/@vivliostyle/theme-techbook/package.json
@@ -3,6 +3,7 @@
   "description": "Techbook (技術同人誌) theme",
   "version": "1.0.0",
   "author": "Vivliostyle <mail@vivliostyle.org>",
+  "main": "theme.css",
   "scripts": {
     "build": "vivliostyle build",
     "preview": "vivliostyle preview",
@@ -46,7 +47,7 @@
     "theme": {
       "name": "Techbook",
       "category": "misc",
-      "style": "./theme.css",
+      "style": "theme.css",
       "topics": [
         "Techbook"
       ]
diff --git a/packages/create-vivliostyle-theme/templates/default/package.json b/packages/create-vivliostyle-theme/templates/default/package.json
index b067dce..cc93e43 100644
--- a/packages/create-vivliostyle-theme/templates/default/package.json
+++ b/packages/create-vivliostyle-theme/templates/default/package.json
@@ -3,6 +3,7 @@
   "description": "{{description}}",
   "version": "0.1.0",
   "author": "{{contact}}",
+  "main": "theme.css",
   "scripts": {
     "build": "vivliostyle build",
     "preview": "vivliostyle preview",
@@ -37,7 +38,7 @@
     "theme": {
       "name": "{{capital name space=true}}",
       "author": "{{contact}}",
-      "style": "./theme.css",
+      "style": "theme.css",
       "category": "{{category}}",
       "topics": []
     }