Just discovered something, not sure if it's bug or not but I'm just reporting it to see if anyone can confirm this issue with me or no:

I have a custom toolbar on my mIRC script which uses /toolbar. If i set any background picture or even a single color background, then the separators are disappearing. However, without any backgrounds it looks fine...
Take a look at this screen shot

And maybe this could be useful: I'm using mIRC v7.19 under Windows 7 Ultimate 32bit.

EDIT: I just installed v7.1 and i see it's the same. So, from v7+ toolbar acts like this.

If I recall, this is intentional. There was a report that separators should not show in switchbar when a background is used, which was implemented in the 7.0x betas and might be related to this change. In the switchbar, separators disappear when a background is used, so the behaviour in the toolbar was likely changed to make it consistent.

Indeed, this is an official change as documented in versions.txt:

Oh i see. However i think it shouldn't be like this, separators must be shown anyway...
Thanks for your reply.

Must agree with you there - I think it was nice and less cumbersome to have each network and it's channels separated from each subsequent network. I would like to see this back and have the option to disable it

How is it "less" cumbersome? Perhaps you're missing the point of the change.

Enabling a background does not disable network separation, it just removes the tiny 1px wide lightgray separator bars that are rendered in place of the actual separations. When the background is on, there is simply an extra black space in the place of the divider. The order of networks does not change.

There is a specific reason why it is disabled. Please see the URL I referenced detailing why it needed to be removed.

The dividers are 1px gray lines, typically with a white shadow. This clashes heavily with dark backgrounds (see the ref url for an example), creating a very weird visual look. Instead of mIRC performing complicated processing to make these dividers blend into the background colour, he simply removed them.