Theme toggle uses text characters instead of SVG icons

[MelbourneDeveloper]

Problem

The plugin's theme toggle button uses plain text characters:

<span class="theme-icon-light">☀</span>
<span class="theme-icon-dark">☾</span>

The dart_node site uses proper SVG icons for the sun/moon which render consistently across all platforms and browsers.

Impact

• Text characters render differently across operating systems
• They don't scale as cleanly as SVGs
• They lack the visual polish of purpose-built icons

Suggestion

Use SVG icons matching the language switcher button style (which already uses SVG in the dart_node site).