This is an excellent quick solution, @francescospissu 👍 I will try it out in action.

@davegarthsimpson, could you please help review the effective code change? Thank you!

Serial Monitor drop-down menu selector hover and selected state.

✅

(I have noticed that the baud dropdown does not have the right border. Not critical.)

Board selector border.

✅

Board selector hover and selected state.

✅

Buttons hover and focus state.

✅

(I have noticed that the settings and 3rd party URLs dialog have different OK/Cancel buttons orders. Not critical.)

Side bar elements (Sketchbook etc.) hover and selected state.

✅

Toolbar buttons hover and selected state.

✅

Quick inputs hover and selected state.

✅

I went through the list of changes, and they look good to me. I found a few places we might want to improve.

• Install/Uninstall button on hover in boards and libs manager view:

Thank you for testing it @kittaakos!

• Sometimes buttons have an orange-ish focus style, sometimes a white one, and sometimes they don't have any. I could not figure out what's the difference

This difference is also present in the other themes:

• The buttons without border are the primary buttons, with the default/dark theme they have a background color but no border. Maybe for the HC theme we should add a border to highlight it.

• Buttons with the white border are secondary buttons.

• The orange focus style is due to the primary button already active when opening the settings dialog, it happens also with the other themes:

• Install/Uninstall button on hover in boards and libs manager view:

I'll provide a fix for this.

Since no one has complained in the past I was expecting to see the restore of the previous version of the adapted HC theme before the introduction of our latest Theme related PR that made us change how some variables are handled.
The previous state of HC theme in Arduino IDE

But even if we have gone for this approach that emulate how vscode handle the HC theme I believe the final result is usable overall, apart from minor issues highlighted by @kittaakos .

The only thing that I would change to align with the visual hierarchy of other Arduino themes is to restore the primary button entity on toolbar for verify, upload and debugger:

Compared to the current one of this PR:

Since no one has complained in the past I was expecting to see the restore of the previous version of the adapted HC theme before the introduction of our latest Theme related PR that made us change how some variables are handled.

These changes have been done taking into account what is reported in this issue:

UI is broken (missing hover status in dropdowns and in the sketchbook, primary buttons background is transparent, board selector has no border, etc...)

About

The only thing that I would change to align with the visual hierarchy of other Arduino themes is to restore the primary button entity on toolbar for verify, upload and debugger:

I'll revert this behavior.

It's looking great. All the issues I reported in #1265 (comment) were fixed. ✅

I found only this one:

Thanks for testing it @kittaakos! I fixed the notification hover status by adding a dashed outline (in Theia Blueprint it works like this):

The Command Palette has no border, and it has the same background as the editor. Also, the elements' hover border is a little off.

Arduino IDE 2:

VS Code

Hovering over an item of the board selector, three separate rectangles show up to highlight the item. I think a single rectangle enclosing the whole item would be better (and more similar to the other themes)

High Contrast

Light

Thanks for testing it @AlbyIanna! I implemented your suggestions here. This is the result:

Boards selector hover:

Command palette border: