If you open the .svg file, try adding the fill attribute to it. As an example: <svg fill="white"
As I understand from what you are trying to do, you are setting an image with SVG properties, such as fill, however this must be done directly to the SVG itself.
Here is another example of a plain circle outline, filled with red.
<svg xmlns="http://www.w3.org/2000/svg" width="80" height="80" viewBox="0 0 24 24" fill="red" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="feather feather-circle"><circle cx="12" cy="12" r="10"></circle></svg>
For any other reasons, consider replacing <img src="Resources/iconmonstr-arrow-65.svg" alt="arrow"> simply with the svg itself - although its not the most ideal method, such as:
<div class="expand-button" onclick="toggleExpand()">
<svg xmlns="http://www.w3.org/2000/svg" width="80" height="80" viewBox="0 0 24 24" fill="red" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round" class="feather feather-circle"><circle cx="12" cy="12" r="10"></circle></svg>
</div>