Fix filled EuiButton in EuiControlBar

[andreadelrio]

Summary

Before

Now

And here's how filled EuiButtons look in all four themes

Fixes #2768

Checklist

• Check against all themes for compatibility in both light and dark modes
  - [ ] Checked in mobile
  - [ ] Checked in IE11 and Firefox
  - [ ] Props have proper autodocs
  - [ ] Added documentation examples
  - [ ] Added or updated jest tests
  - [ ] Checked for breaking changes and labeled appropriately
  - [ ] Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[snide]

Can you include an example of the button in one of the docs examples. Stylistic feedback, so I'll let @ryankeairns chime in, but I don't think you need the border in the filled version. That way it's closer to the feel of the other buttons on the page.

[ryankeairns]

+1 for dropping the border on the filled versions.

[andreadelrio]

I removed the border from the fill button and I added one of them to our examples.

It looks like this

[snide]

Small CL suggestion for clarity. Otherwise LGTM. Thanks!

[snide]

Suggested change

	- Fixed filled `EuiButton`s in `EuiControlBar` ([#2781](https://github.com/elastic/eui/pull/2781))
	- Fixed the display of `EuiButton` within `EuiControlBar` when `fill={true}` to be more consistent with other buttons ([#2781](https://github.com/elastic/eui/pull/2781))

[ryankeairns]

I pulled this up locally and quickly got sidetracked into other conversations, but I'm curious what colors this is using for buttons. They look lighter than our standard dark-mode button colors 🤔

The borderless approach looks great!

[ryankeairns]

Looked a little more and I think my comment might be opening a rabbit hole that can be addressed at a later time. The control bar background is darker with more vibrant button colors in dark mode, while we use a lighter background and lighter button colors in light mode. I wonder about that latter combination, but could punt on that for now.

[snide]

I noticed that too. I think we should address that separately. To be honest, this is something better solved by a JS theming layer, otherwise we're writing waaaaay too much sass in inception like ways. A little is fine, but if we know (which we do) that this will primarily be for the primary / secondary fill buttons, I think we should be OK letting this go till the theming layer is more robust (for this theme within a theme stuff)