Add themes support

Usage:

  const editor = new RibbitEditor({
    themes: [
        { name: 'dark', features: { sourceMode: false } },
        { name: 'minimal', tags: minimalTags },
    ],
    currentTheme: 'dark',
  });

The built-in theme is 'ribbit-default' and is always available.
Additional themes from the themes array are registered on top.

src/static/ribbit-core.css

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+/*
+ * ribbit-core.css — functional editor styles. Always load this.
+ * These styles control editor state visibility and behavior.
+ * They should not be overridden by themes.
+ */
+
+#ribbit {
+    display: none;
+}
+
+#ribbit.loaded {
+    display: block;
+}
+
+#ribbit.edit {
+    font-family: monospace;
+    white-space: pre;
+}
+
+#ribbit.wysiwyg .md {
+    opacity: 0.5;
+}

src/static/themes/ribbit-default/theme.css

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+/*
+ * default.css — the default ribbit theme.
+ * Provides basic aesthetic styling for rendered markdown content.
+ * Replace this file with your own theme to customize the look.
+ */
+
+@import "../ribbit-core.css";
+
+a {
+    text-decoration: none;
+}
+
+blockquote {
+    margin-left: 30px;
+    font-size: 1.3em;
+    font-style: italic;
+    color: #555;
+}
+
+table {
+    width: 100%;
+}
+
+th {
+    border-bottom: 1px solid #000;
+    padding: 3px;
+}
+
+th, td {
+    padding: 2px;
+}
+
+table td table {
+    max-width: 95%;
+}
+
+pre {
+    border: 1px dashed black;
+    border-radius: 5px;
+    padding: 10px;
+    margin: 5px;
+    background: #EEE;
+}
+
+code {
+    display: inline-block;
+    border: 1px dashed black;
+    border-radius: 5px;
+    padding: 5px;
+    background: #EEE;
+    margin: 3px;
+}