Extension logo is unreadable in dark mode

[louie-github]

Hello there! I've tested out this extension in both Firefox Nightly (with the new Proton UI) and Chrome Dev, and both seem to be working (with Chrome via "Load unpacked" and still needing a fix described in #8). Thank you for your work!

Although, one thing I have noticed is that the extension logo is almost unreadable when the browser theme is set to dark mode.

Here's an example in Firefox Nightly: The logo is only visible when hovered over, but in the regular menu, it looks almost invisible.

Here's an example in Chrome Dev's Extensions menu:

I have yet to try this on Firefox's old, stable UI, and I will update once I have time to reinstall Firefox Developer Edition and check the logo.

My suggestion is to fill the logo with white so it remains readable in both light and dark modes. Once again, thank you!