Visual customization in Linux can be frustrating but it can be done easily if you know where to look.
Ubuntu MATE shares many icons with gnome and the hicolor icon themes having itself originated from the gnome2 desktop before Unity was developed by Canonical. We have now come full circle (because puns). The MATE developers have done a fantastic job in [re]creating the efficient Mutiny desktop paradigm but your theme may be using the default gnome foot icon which isn't the nicest aesthetic...
In fact, there have been several requests along this not-a-bug-its-a-feature issue: here and here. Be warned, playing around in the dconf-editor or gconf-editor (gsettings) will NOT change the icon. You must manually override it with an image of your own making.
The Brisk menu, which is a joint project between Solus and MATE, specifically uses the Adwaita icon set as a fallback in case the current theme doesn't have a present Brisk icon.
Brisk uses a combination of both png and svg icons titled "start-here-symbolic." For example, if your MATE desktop panel is set to 64px, the location would be in:
If you were using a panel size of 48px the location would be:
If you do not find such a folder, your current theme might be relying on the .svg variant located in the scalable folder at:
One can always search graphically by using the file name "start-here" within the Adwaita folder and visually identify the solid gray gnome foot that matches your Brisk launcher. Use the open file location right click option to review where the file actually lives and choose the icon size closest to your own panel size.
As you can see from our image we are using a panel size of 64px with the United Gnome theme. This theme emulates and combines the window and controls style of Unity8 with long-standing Ubuntu icon theme Humanity-Dark.
The launcher itself was created using Inkscape from the vanilla Humanity launcher with a small highlight per Sam Hewitt's original Suru style guidelines.
We simply opened the appropriate
/usr/share/icons/64x64/places/ location as an administrator (use your sudo password when prompted) and renamed the original "start-here-symbolic" icon to something like
start-here-symbolic.png.bak. That will keep the original right next to your customized replacement.
After doing so, mouse over the Brisk menu; it will change to the default error symbol, a gray box with a red error sign. If it does not, your current theme is not using that particular icon for the menu and you'll have to search again to find the correct location.
We copied the original file, renamed it back to
start-here-symbolic.png, editing it with GIMP. Be sure you are still using sudo permissions. The file is owned and read by the root user with read-only for others. With success, your icon will change to your preferred version as soon as you mouse over it following your image save.