This problem happens with gtk3 branch (d47c8957a1)

(In reply to comment #0)
> The applets only change their background color when the style
> of the solid color selected is 100% opaque, but they should
> change theirs colors when the style bar is moved to other 
> values.
> 
> How to reproduce it:
> 1. In Panel Properties dialog (right click on a panel)
> 2. Solid color
> 3. Set style bar in any position, except Opaque.
> 
> Result:
> Only the background of the panel change the color, but the applet
> stays with the same (theme) background color.
> 
> Expected Result:
> The applets should change their background color to match the color
> of the panel background.

I can't reproduce it, it works for me as expected.

> As a side note: The following warning appears when setting the Solid
> color for the background of the panel:
> 
> ** (gnome-panel:13414): WARNING **: couldn't get background pixmap

This is normal, when the opacity changes the background pixmap is created and destroyed several times, so it's posssible that when the panel tries to get the pixmap it has been destroyed, but get_pixmap will be called again with the new pixmap so it's not a problem. 

> It seems the background is trying to set a pixmap, when it should not.
> When loading the panel again, the message appears once per panel and 
> per applet.

hmm, if it always happens that might explain why the applet background doesn't change for you.

libpanel-applet has changed, so make sure your applets are using the new version.

(In reply to comment #3)
> libpanel-applet has changed, so make sure your applets are using the new
> version.

I always run gnome-panel after installing it.  To be sure, I also
delete anything with panel in my directory:

$ find ~/code/gnome3/install -name '*panel*' -exec rm {} \;

FWIW, I run gnome-panel using a test user.

Created attachment 178799 [details]
Screenshot with gnome-panel changing background

Here is the screenshot when I am changing the background
of a panel using solid color, but with a value different 
than opaque.

(In reply to comment #4)
> (In reply to comment #3)
> > libpanel-applet has changed, so make sure your applets are using the new
> > version.
> 
> I always run gnome-panel after installing it.  To be sure, I also
> delete anything with panel in my directory:
> 
> $ find ~/code/gnome3/install -name '*panel*' -exec rm {} \;
> 
> FWIW, I run gnome-panel using a test user.

~/code/gnome3/install/libexec$ ldd *applet  | grep panel
	libpanel-applet-4.so.0 => /home/gpoo/code/gnome3/install/lib/libpanel-applet-4.so.0 (0x00725000)
	libpanel-applet-4.so.0 => /home/gpoo/code/gnome3/install/lib/libpanel-applet-4.so.0 (0x001e2000)
	libpanel-applet-4.so.0 => /home/gpoo/code/gnome3/install/lib/libpanel-applet-4.so.0 (0x00bf0000)

And /~/code/gnome3/install/lib/libpanel-applet-4.so.0 points
to:
2011-01-19 15:02  ~/code/gnome3/install/lib/libpanel-applet-4.so.0.0.5

Compiled some minutes ago, to retest it.

This is my current desktop using gnome-panel with gtk3:

http://people.gnome.org/~carlosgc/gnome-panel-3.png

The window-menu list has a border, and the systray doesn't work at all, but other than that we are close to have a usable panel with gtk3 :-) 

It seems you are using Xephyr or something, is it possible that your applets run in a different display than the panel?

Are you using gnome-applets from current git master? I changed a couple of things yesterday.

(In reply to comment #7)
> This is my current desktop using gnome-panel with gtk3:
> 
> http://people.gnome.org/~carlosgc/gnome-panel-3.png
> 
> The window-menu list has a border, and the systray doesn't work at all, but
> other than that we are close to have a usable panel with gtk3 :-) 
> 
> It seems you are using Xephyr or something, is it possible that your applets
> run in a different display than the panel?

Yes, I was using Xephyr.  However, after recompiling gnome-panel, gnome-applets
and some dependencies (ie gtk+) I also tried using just a plain X server.  That is:

1. Log in in tty1 with my test user 'gnome'.
2. Run:
   $ startx -- :1

And the .xinitrc is the following:
------------------------------------------------------
#!/bin/bash

export GNOME=/home/gpoo/code/gnome3/install
 
export GDK_USE_XFT=1
export XDG_DATA_DIRS=$GNOME/share:$XDG_DATA_DIRS
export XDG_CONFIG_DIRS=$XDG_CONFIG_DIRS:$GNOME/etc/xdg

echo $XDG_DATA_DIRS
echo $XDG_CONFIG_DIRS

jhbuild run gnome-session --session classic-gnome
------------------------------------------------------

> Are you using gnome-applets from current git master? I changed a couple of
> things yesterday.

I also tried the current master (6fd55674c).

FWIW, only a custom launcher changed the background properly and
a "Logout" applet changed their background when I set the 
traslucency.  BTW, I think the "Logout" applet is a ubuntu thing.

FWIW, I tried also with --with-in-process-applets=all.

Ok, it happens when there is not image set as background. For instance,
if I try 'xsetroot -solid black' neither works (because it is not an 
image).  It is a very particular corner case and I do not think it worths
to check it.

However (and that is the reason I am reopening the bug): The height of
the panel changes if I toggle between:
- None (use system them)
- Solid color

It could be because the default them have a border.  It also happens in
2.30.2.  If that is the explanation, feel free to mark it as not a bug.

(In reply to comment #10)
> However (and that is the reason I am reopening the bug): The height of
> the panel changes if I toggle between:
> - None (use system them)
> - Solid color
> 
> It could be because the default them have a border.  It also happens in
> 2.30.2.  If that is the explanation, feel free to mark it as not a bug.

Yes, that's because there's a frame when we use the system theme.