I would love to get rid of this annoying line also. Its the last thing needed to make Windows 10 OK in my opinion. The start menu can be fixed up with Classic Shell and the rest looks & works fine.
With the help from the Windows Style Builder forum I was able to get rid of those lines.
It is a custom theme if you want to use it you need to install UxStyle which I have provided in the folder below with the theme.
This is how to open up the control panel if you don't know how Control Panel - Open in Windows 10 - Windows 10 Forums
First you need to turn off the UAC User Account Control (UAC) - Change Settings in Windows 10 - Windows 10 Forums slide the slider all the way down (Be sure to put it back to where you had it when done installing everything and reboot), and then afterwards reboot your computer, and then install UxStyle and then restart your computer again, put the NoLine folder and the NoLine.theme file in C:/Windows/resources/themes folder, then double-click on the NoLine.theme file and if all went well you should have no lines under the open windows and programs, this only works on the taskbar when it's on the bottom.
Does anyone know what the different line colors represent? My Outlook has a red line while other programs are underline in blue.
I think its a nice feature. It doesn't effect anything. I think its supposed to be that way I don't think there is anyway to change it.
The taskbar lines are horrible visually - they're only necessary if you have pinned shortcuts, and need to distinct them from the other icons - if you have old-style taskbar view, and a dark taskbar background, they literally glow, distracting the desktop, and without any real use (all the taskbar items are underlined).
Here you go.
Works on 10586 only
Turn off UAC, reboot.
User Account Control (UAC) - Change Settings in Windows 10 - Windows 10 Forums
Do one at a time and then reboot in between.
Right click on UxStyle_0242_x86_x64_preview.exe run as administrator, install, reboot.
Right click on UltraUXThemePatcher_3.0.4.exe run as administrator, install, reboot.
Run take ownership add to context menu windows 10.
Created by: Shawn Brink
Take Ownership - Add to Context Menu in Windows 10 - Windows 10 Forums
Credit go to Mr. GRiM on this way of installing themes.
1. Rename the default C:\Windows\Resources\Themes\aero\aerolite.msstyles to aerolite.msstyles.bak (right click on it and take ownership then rename)
2. Make a new folder in C:\Windows\Resources\Themes\aero\ called aerolite.msstyles
3. Put the No_Line_1511 theme folder in the aerolite.msstyles folder C:\Windows\Resources\Themes\aero\aerolite.msstyles\
4. Put the No_Line_1511.theme file in C:\Windows\Resources\Themes
Select the No_Line_1511 theme from the personalization menu.
Turn on UAC, reboot.