add custom-variables

snippets/custom-variables.md

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+### Custom variables
+
+CSS variables that contain specific values to be reused throughout a document.
+
+#### CSS
+
+```css
+:root {
+    --some-color: #da7800;
+    --some-keyword: italic;
+    --some-size: 1.25em;
+    --some-complex-value: 1px 1px 2px WhiteSmoke, 0 0 1em SlateGray , 0 0 0.2em SlateGray;
+}
+```
+
+#### Demo
+
+<!-- You must create a `snippet-demo` parent block and use it as a namespace with BEM syntax. -->
+
+<div class="snippet-demo">
+  <div class="snippet-demo__custom-variables">
+    <p>CSS is awesome!</p>
+  </div>
+</div>
+
+<!-- Add your style rules here. -->
+
+<style>
+:root {
+    --some-color: #686868;
+    --some-keyword: italic;
+    --some-size: 1.25em;
+    --some-complex-value: 1px 1px 2px WhiteSmoke, 0 0 1em SlateGray , 0 0 0.2em SlateGray;
+}
+
+.snippet-demo__custom-variables p{
+    color: var(--some-color);
+    font-size: var(--some-size);
+    font-style: var(--some-keyword);
+    text-shadow: var(--some-complex-value);
+}
+
+</style>
+
+#### Explanation
+
+<!-- Use a step-by-step (ordered) list if possible. Keep it concise. -->
+Bla bla bla
+
+#### Browser support
+
+<!-- Use the checkmark or the warning emoji, see the existing snippets. -->
+
+<span class="snippet__support-note">✅ No caveats.</span>
+
+<!-- Whenever possible, link a `caniuse` feature which allows the browser support percentage to be displayed.
+If no link is provided, it defaults to 99+%. -->
+
+* https://caniuse.com/#feat=some-feature