In VS2019 go to Extensions -> Manage Extensions -> Online -> Visual Studio Market Place -> Tools -> Themes ... and you'll have plenty to choose and it includes a link to preview called "More information".

Oh! to use the downloaded theme you should go to Tools -> Options -> Environment -> General and inside it Color Theme.

In VS code 'User Settings', you can edit visible colours using the following tags (this is a sample and there are much more tags):

"workbench.colorCustomizations": {
    "list.inactiveSelectionBackground": "#C5DEF0",
    "sideBar.background": "#F8F6F6",
    "sideBar.foreground": "#000000",
    "editor.background": "#FFFFFF",
    "editor.foreground": "#000000",
    "sideBarSectionHeader.background": "#CAC9C9",
    "sideBarSectionHeader.foreground": "#000000",
    "activityBar.border": "#FFFFFF",
    "statusBar.background": "#102F97",
    "scrollbarSlider.activeBackground": "#77D4CB",
    "scrollbarSlider.hoverBackground": "#8CE6DA",
    "badge.background": "#81CA91"
}

If you want to edit some C++ color tokens, use the following tag:

"editor.tokenColorCustomizations": {
    "numbers": "#2247EB",
    "comments": "#6D929C",
    "functions": "#0D7C28"
}

Thanks to Dorin Botan's comment, I just needed to do input @builtin to the Extensions panel:

And then from the Setting icon, enable the default themes again.