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That's strange, it should just be tied to the main color picker - does it update if you select a new secondary that way?

The way custom brushes work (that is what the extension uses) makes it impossible for this extension at least to round the pattern. Someone requested this previously and when I tried to get it working I'm pretty sure there would always be a weird gap where the corners of the square patterns would be.

Maybe in the future if aseprite supports custom tools I could make a dithering brush tool that would work much better. For now though I don't think it is possible without modifying source code.

This is now possible as of 2.0.1 by setting the pattern size increment to a value less than zero. For example, -2 will give you a 2x2 brush. That is unfortunately as small as it can go for now. Also note that the -1 increment will not tile the same way as even values.

I'm not able to test on rc6 without building from source. I assume you have tried this but just to confirm - tiling only really works with even brush sizes. If tiling is not otherwise working I am not sure what to tell you since all the extension is doing is creating a new variant of the built-in custom brush feature. I will investigate this further once I have rc6.

Just updated the extension, there is now a check box to automatically apply changes made in the dialog.

I am glad you are finding the extension useful.
I took a closer look at the api/code and I am not sure how possible it is to have rounded brushes. The main issue is that custom brushes in aseprite are based on having a fixed image that gets tiled - if I make the image circular, the pattern becomes circular without affecting the actual brush shape. To see what I mean you can try drawing a circular pattern and then selecting the pattern and pressing CTRL+B to create a custom brush for it.
As far as pressure sensitivity goes, I don't really have any way to test that and I doubt the API provides any way to detect it.
Maybe in the future if the API becomes more advanced I can create a custom brush tool as opposed to a menu that creates custom brushes.

That should be possible. I would probably also add a check box to enable auto-updating.

Those are good ideas, I could definitely figure out rounded brushes. Pressure sensitivity might take a bit longer though.

From the top menu: 
Edit > Dithering Brushes
it should be below 'Shift' and above 'New Brush'

What is your aseprite version?

It is possible that it will only work for 1.3 and above, at least that's the version I use and have it working on.

If double clicking the file didn't work you can go to Edit / Preferences / Extensions then click where it says add extension and find where you downloaded it.