Tab Mix Plus Closed Tabs List Icon

### Is your feature request related to a problem?

Hi, if it's not too much to ask, can you change the icon for Tab Mix Plus Closed Tabs List?


### The solution I'd like

Please add this 👉 `#tm-undoCloseList`

### Screenshots or video

It can be enabled in Events > Tab Features 👇


### Alternatives I've considered

I tried adding a custom icon in userChrome.css and leptonChrome.css, but it won't show up.
It might need a bit more tweaking just like `#context_openANewTab` 😄

### Additional context

N/A

Start by locating the SCSS styling for the Tab Mix Plus Closed Tabs List and compare it with the requested #tm-undoCloseList selector and the existing #context_openANewTab handling. Enable the feature through Events > Tab Features in Firefox and verify that the requested icon appears and remains compatible with the theme.

Written by the indexing model from the issue text.