Add background pattern snippets

snippets/polka-dot-pattern.md

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+---
+title: Polka dot background pattern
+tags: visual,intermediate
+---
+
+Creates a polka dot background pattern.
+
+- Use `background-color` to set a black background.
+- Use `background-image` with two `radial-gradient()` values to create two dots.
+- Use `background-size` to specify the pattern's size and `background-position` to appropriately place the two gradients.
+- **Note:** The fixed `height` and `width` of the element is for demonstration purposes only.
+
+```html
+<div class="polka-dot"></div>
+```
+
+```css
+.polka-dot {
+  width: 240px;
+  height: 240px;
+  background-color: #000;
+  background-image: radial-gradient(#fff 10%, transparent 11%),
+    radial-gradient(#fff 10%, transparent 11%);
+  background-size: 60px 60px;
+  background-position: 0 0, 30px 30px;
+  background-repeat: repeat;
+}
+```