There is no option to change the appearance for the active window in lxpanel's Task Bar (Window List) plugin. You can patch the lxpanel source file launchtaskbar.c in the most recent version, LX Panel 0.7.1. - LXDE - Lightweight X Desktop Environment - Browse /LXPanel (desktop panel) at SourceForge.net
For example, you could make the active window use bold font while using the flat button style:
Find line 1589 of launchtaskbar.c which reads:
gboolean bold_style = (((tk->entered_state) || (tk->flash_state)) && (tk->tb->flat_button));
gboolean bold_style = (((tk->entered_state) || (tk->flash_state) || (tk->focused)) && (tk->tb->flat_button));
The only change is that
tk->focused was added to
bold_style to check if the window for that item is focused.
Ultimately, it require some knowledge of coding in C to add custom functionality to the appearance. If you're not familiar with patching, research the topics HowTo Apply a Patch File To My Linux / UNIX Source Code and How to Compile and Install Software from Source Code on Linux.
The above example is from the following patch which appears to be outdated since it targets the file name taskbar.c instead of launchtaskbar.c. - LXDE - Lightweight X Desktop Environment / Feature Requests / #213 [lxpanel][taskbar] highlight active task with flat buttons
Update: I compiled a patched source with the above code on lxpanel-0.6.1 and it was glitchy. It kept reloading lxpanel. You might try an SE network that's more oriented toward coding to figure out how to customize it this way.