LXDE uses openbox as windowmanager
openbox has a config-file in
~/.config/openbox/lubuntu-rc.xml
(this is a simple text-file, so use an appropriate text-editor for changes, like vim)
this file has a -normaly- empty section called
....
....
but with an example of possible settings there.
I dont use steam, but to disable the appearance of xosview in the panel
and display it without window-decorations, i did add the following there:
<application name="xosview" title="xosview" type="normal" class="XOsview" role="">
<decor>no</decor>
<desktop>all</desktop>
<focus>no</focus>
<layer>below</layer>
<!-- above, below -->
<skip_pager>yes</skip_pager>
<skip_taskbar>yes</skip_taskbar>
</application>
the entry for skip_pager and skip_taskbar may be whats necessary for the steam-program. You have to check the right program name, maybe look it up with lxtask.