diff --git a/src/importers/__tests__/sassTildeImporter.ts b/src/importers/__tests__/sassTildeImporter.ts
new file mode 100644
index 0000000..cbf6edd
--- /dev/null
+++ b/src/importers/__tests__/sassTildeImporter.ts
@@ -0,0 +1,53 @@
+import sass from 'sass';
+import { sassTildeImporter } from '../sassTildeImporter';
+
+describe('importers / sassTildeImporter', () => {
+  const source = 'src/importers/sassTildeImporter.ts';
+  const done = (data: sass.ImporterReturnType) => void data;
+
+  it('should return null when not a tilde import', () => {
+    expect(sassTildeImporter('color.scss', source, done)).toBeNull();
+  });
+
+  it('should return null when node module does not exist', () => {
+    expect(
+      sassTildeImporter('~made_up_module/color.scss', source, done),
+    ).toBeNull();
+  });
+
+  it('should resolve file from node_modules', () => {
+    expect(
+      sassTildeImporter('~bootstrap/scss/bootstrap', source, done),
+    ).toMatchObject({ file: 'node_modules/bootstrap/scss/bootstrap.scss' });
+  });
+
+  it('should resolve sass partial from node_modules', () => {
+    // explicit
+    expect(
+      sassTildeImporter('~bootstrap/scss/_grid.scss', source, done),
+    ).toMatchObject({ file: 'node_modules/bootstrap/scss/_grid.scss' });
+    expect(
+      sassTildeImporter('~bootstrap/scss/_grid', source, done),
+    ).toMatchObject({ file: 'node_modules/bootstrap/scss/_grid.scss' });
+    // implicit
+    expect(
+      sassTildeImporter('~bootstrap/scss/grid.scss', source, done),
+    ).toMatchObject({ file: 'node_modules/bootstrap/scss/_grid.scss' });
+    expect(
+      sassTildeImporter('~bootstrap/scss/grid', source, done),
+    ).toMatchObject({ file: 'node_modules/bootstrap/scss/_grid.scss' });
+  });
+
+  it('should resolve .css files', () => {
+    expect(
+      sassTildeImporter('~bootstrap/dist/css/bootstrap-grid.css', source, done),
+    ).toMatchObject({
+      file: 'node_modules/bootstrap/dist/css/bootstrap-grid.css',
+    });
+    expect(
+      sassTildeImporter('~bootstrap/dist/css/bootstrap-grid', source, done),
+    ).toMatchObject({
+      file: 'node_modules/bootstrap/dist/css/bootstrap-grid.css',
+    });
+  });
+});
diff --git a/src/importers/sassTildeImporter.ts b/src/importers/sassTildeImporter.ts
index af1cf99..ed8be96 100644
--- a/src/importers/sassTildeImporter.ts
+++ b/src/importers/sassTildeImporter.ts
@@ -19,11 +19,28 @@ export const sassTildeImporter: sass.Importer = (
   // for an import of the form ~@foo/bar/baz(.(scss|sass))?
   const nodeModSubpath = path.join('node_modules', rawImportPath.substring(1));
   const subpathsWithExts: string[] = [];
-  if (nodeModSubpath.endsWith('.scss') || nodeModSubpath.endsWith('.sass')) {
+  if (
+    nodeModSubpath.endsWith('.scss') ||
+    nodeModSubpath.endsWith('.sass') ||
+    nodeModSubpath.endsWith('.css')
+  ) {
     subpathsWithExts.push(nodeModSubpath);
   } else {
     // Look for .scss first.
-    subpathsWithExts.push(`${nodeModSubpath}.scss`, `${nodeModSubpath}.sass`);
+    subpathsWithExts.push(
+      `${nodeModSubpath}.scss`,
+      `${nodeModSubpath}.sass`,
+      `${nodeModSubpath}.css`,
+    );
+  }
+
+  // Support sass partials by including paths where the file is prefixed by an underscore.
+  const basename = path.basename(nodeModSubpath);
+  if (!basename.startsWith('_')) {
+    const partials = subpathsWithExts.map((file) =>
+      file.replace(basename, `_${basename}`),
+    );
+    subpathsWithExts.push(...partials);
   }
 
   // Climbs the filesystem tree until we get to the root, looking for the first