Available in version 3.6 or later.
You can add buttons to a toolbar using Shortcuts advanced setting.
To customize toolbar:
Example of customization:
Shortcuts [
    [
        Cmd = CmdPrevTab
        ToolbarText = Prev Tab
    ]
    [
        Cmd = CmdNextTab
        ToolbarText = Next Tab
    ]
    [
        Name = Send By Mail
        Cmd = CmdSendByEmail
        Key = Shift + M
        ToolbarText = ✉
    ]
    [
        Cmd = CmdNavigateBack
        ToolbarText = ←
    ]
    [
        Cmd = CmdNavigateForward
        ToolbarText = →
    ]
]
Explanation:
If you provide Name, it’ll be available in Command Palette.
See customizing shortcuts for more complete docs on Shortcuts advanced setting.

Using icons #

Ideally we would allow SVG icons but that’s not easy to implement.
Instead, you can use Unicode symbols supported by Windows’ Segoe UI font: http://zuga.net/articles/unicode-all-characters-supported-by-the-font-segoe-ui/
To find a symbol you can search for example for arrow and then copy & paste the symbol (e.g. ) into settings file.

edit