diff --git a/scripts/build-plugins/rollup-plugin-build-themes.js b/scripts/build-plugins/rollup-plugin-build-themes.js
new file mode 100644
index 00000000..74fe4daf
--- /dev/null
+++ b/scripts/build-plugins/rollup-plugin-build-themes.js
@@ -0,0 +1,174 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+async function readCSSSource(location) {
+    const fs = require("fs").promises;
+    const path = require("path");
+    const resolvedLocation = path.resolve(__dirname, "../../",  `${location}/theme.css`);
+    const data = await fs.readFile(resolvedLocation);
+    return data;
+}
+
+async function appendVariablesToCSS(variables, cssSource) {
+    return cssSource + `:root{\n${Object.entries(variables).reduce((acc, [key, value]) => acc + `--${key}: ${value};\n`, "")} }\n\n`;
+}
+
+function parseBundle(bundle) {
+    const chunkMap = new Map();
+    const assetMap = new Map();
+    let runtimeThemeChunk;
+    for (const [fileName, info] of Object.entries(bundle)) {
+        if (!fileName.endsWith(".css")) {
+            continue;
+        }
+        if (info.type === "asset") {
+            /**
+             * So this is the css assetInfo that contains the asset hashed file name.
+             * We'll store it in a separate map indexed via fileName (unhashed) to avoid
+             * searching through the bundle array later.
+             */
+            assetMap.set(info.name, info);
+            continue;
+        }
+        if (info.facadeModuleId?.includes("type=runtime")) {
+            /**
+             * We have a separate field in manifest.source just for the runtime theme,
+             * so store this separately.
+             */
+            runtimeThemeChunk = info;
+            continue;
+        }
+        const location = info.facadeModuleId?.match(/(.+)\/.+\.css/)?.[1];
+        if (!location) {
+            throw new Error("Cannot find location of css chunk!");
+        }
+        const array = chunkMap.get(location);
+        if (!array) {
+            chunkMap.set(location, [info]);
+        }
+        else {
+            array.push(info);
+        }
+    }
+    return { chunkMap, assetMap, runtimeThemeChunk };
+}
+
+module.exports = function buildThemes(options) {
+    let manifest, variants, defaultDark, defaultLight;
+
+    return {
+        name: "build-themes",
+        enforce: "pre",
+
+        async buildStart() {
+            const { manifestLocations } = options;
+            for (const location of manifestLocations) {
+                manifest = require(`${location}/manifest.json`);
+                variants = manifest.values.variants;
+                const themeName = manifest.name;
+                for (const [variant, details] of Object.entries(variants)) {
+                    const fileName = `theme-${themeName}-${variant}.css`;
+                    if (details.default) {
+                        // This is one of the default variants for this theme.
+                        if (details.dark) {
+                            defaultDark = fileName;
+                        }
+                        else {
+                            defaultLight = fileName;
+                        }
+                    }
+                    // emit the css as built theme bundle
+                    this.emitFile({
+                        type: "chunk",
+                        id: `${location}/theme.css?variant=${variant}`,
+                        fileName,
+                    });
+                }
+                // emit the css as runtime theme bundle
+                this.emitFile({
+                    type: "chunk",
+                    id: `${location}/theme.css?type=runtime`,
+                    fileName: `theme-${themeName}-runtime.css`,
+                });
+            }
+        },
+
+        async load(id) {
+            const result = id.match(/(.+)\/theme.css\?variant=(.+)/);
+            if (result) {
+                const [, location, variant] = result;
+                const cssSource = await readCSSSource(location);
+                const config = variants[variant];
+                return await appendVariablesToCSS(config.variables, cssSource);
+            }
+            return null;
+        },
+
+        transformIndexHtml(_, ctx) {
+            let darkThemeLocation, lightThemeLocation;
+            for (const [, bundle] of Object.entries(ctx.bundle)) {
+                if (bundle.name === defaultDark) {
+                    darkThemeLocation = bundle.fileName;
+                }
+                if (bundle.name === defaultLight) {
+                    lightThemeLocation = bundle.fileName;
+                }
+            }
+            return [
+                {
+                    tag: "link",
+                    attrs: {
+                        rel: "stylesheet",
+                        type: "text/css",
+                        media: "(prefers-color-scheme: dark)",
+                        href: `./${darkThemeLocation}`,
+                    }
+                },
+                {
+                    tag: "link",
+                    attrs: {
+                        rel: "stylesheet",
+                        type: "text/css",
+                        media: "(prefers-color-scheme: light)",
+                        href: `./${lightThemeLocation}`,
+                    }
+                },
+            ];
+        },
+
+        generateBundle(_, bundle) {
+            const { assetMap, chunkMap, runtimeThemeChunk } = parseBundle(bundle);
+            for (const [location, chunkArray] of chunkMap) {
+                const manifest = require(`${location}/manifest.json`);
+                const compiledVariables = options.compiledVariables.get(location);
+                const derivedVariables = compiledVariables["derived-variables"];
+                const icon = compiledVariables["icon"];
+                manifest.source = {
+                    "built-asset": chunkArray.map(chunk => assetMap.get(chunk.fileName).fileName),
+                    "runtime-asset": assetMap.get(runtimeThemeChunk.fileName).fileName,
+                    "derived-variables": derivedVariables,
+                    "icon": icon
+                };
+                const name = `theme-${manifest.name}.json`;
+                this.emitFile({
+                    type: "asset",
+                    name,
+                    source: JSON.stringify(manifest),
+                });
+            }
+        }
+    }
+}