After upgrading Kubuntu from 15.10 to 16.04, the text really looked ugly: it was darker (even when my theme specified “#ffffff”) and looked imprecise.
In fact, the reason was the default font which was now “Courier New”. The solution, which had nothing to do with the text color (!), was to modify the configuration file (menu File / Preferences / User Settings) by adding an entry like:
"editor.fontFamily": "Liberation Mono"
"editor.fontFamily": "Oxygen Mono"
I struggled to find why it only impacted Kubuntu 16.04 (either KDE and XFCE), and not Ubuntu 16.04 (on Unity) or Ubuntu 15.10 (on KDE): using some VMs I compared (with “strace”) the files loaded by vscode, without any success: it seemed that the files were the same, including the GTK 2 themes.
In fact, I could have found more quickly by using the vscode (Chromium) dev tools (unlock them in the Help menu and press F12): I would have seen which was the font actually used by the editor.