Add theming docs

docs/theming/README.md

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+# Theming WooCommerce Blocks
+
+This page includes all documentation regarding WooCommerce Blocks and themes.
+
+-   [Product grid blocks style update in 2.7.0](./product-grid-270.md)

docs/theming/product-grid-270.md

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+# Product grid blocks style update in 2.7.0
+
+In WC Blocks 2.7.0, some of the styles of the product grid blocks were updated to make the experience more consistent. Below, there are CSS code snippets that can undo those changes.
+
+## Product images
+
+Images in product grid blocks expand to occupy all the available horizontal space if they are small. This change can be undone with this CSS snippet:
+
+```CSS
+.wc-block-grid__products .wc-block-grid__product-image img {
+	width: auto;
+}
+```
+
+## All Products prices
+
+Prices in the _All Products_ block follow the same layout as the other product grid blocks. This can be undone with:
+
+```CSS
+.wc-block-grid__product-price .wc-block-grid__product-price__regular {
+	font-size: 0.8em;
+	line-height: 1;
+	color: #555;
+	margin-top: -0.25em;
+	display: block;
+}
+.wc-block-grid__product-price .wc-block-grid__product-price__value {
+	letter-spacing: -1px;
+	font-weight: 600;
+	display: block;
+	font-size: 1.25em;
+	line-height: 1.25;
+	color: #000;
+	margin-left: 0;
+}
+.wc-block-grid__product-price .wc-block-grid__product-price__value span {
+	white-space: nowrap;
+}
+```