A button can be Shift+drag-dropped between the customised toolbars.

Enhancement for all the default toolbars:
">>" button when truncated by the window width.

Markers toolbar:
I think "Add to Markers" command could be "Add to/Remove from Markers". Thus we can toggle the marker on a word by using the shortcut key without using the mouse.
The color for "Auto Marker" can be customized. (I found the setting. Sorry.)
Option: Don't prompt when deleting a single marker.
Option: Ignore Case by default.

Find toolbar:
Option: Switch the focus to the editor after replaced.

Another suggestion for Find toolbar:

Hope the width of the input boxes can be adjusted by dragging the right edge of the boxes, like how we do on the Google toolbar in the web browser.