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
    ]
]
Explanation:
See customizing shortcuts for more complete docs on Shortcuts advanced setting.

Using icons #

Ideally we would allow SVG icons but that’s not possible.
You can use Unicode symbols supported by 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.

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