Hi, I 've followed your tutorial on customization of maintenance mode page
It’s work great on Default mode, but as soon as I switch to Dev mode it trow the regular exception message. Any solution to just get the Customised Maintenance Modepage in dev mode while building my site?
Not to my knowledge. Developer Mode is used for internal purposes (e.g. Debugging), therefore the system message will always display instead of any mask.
Albeit, I do see the irony of not being able to view the page you’re trying to design whilst in the mode that you would design it in 
But at the end of the day, the static Maintenance Mode page is just basic HTML/CSS anyway so there wouldn’t be any foreseeable difference from previewing the code in a web browser and seeing it in practice. On a side note, I use https://codepen.io/pen to mock up most of my HTML/CSS/JS designs. It’s a great time-saver.