This is probably not a duplicate of  issue 819781 , and AFAIK cannot be bisected because the new tab page is an externally maintained resource. I guess the component is "UI>Browser>NewTabPage". To reproduce you need to have a display larger than 1920x1080, a maximized browser window, and a standard (not 4K) theme, which is 99% of themes, for example:
https://chrome.google.com/webstore/detail/cartoon-network-theme/bcmcfeibpmflkfhibpdoocejilahahnn
https://chrome.google.com/webstore/detail/hokusais-wave-theme-1920/hphdhdhlleolbgddlhbpiiofonbfijph

Looking at the CSS of the new tab page source code I see it lacks "body {background-size:cover}" which would have fixed the issue.

Unable to reproduce this issue o reported version 65.0.3325.162 using Windows 10 HIDPI laptop with screen resolution 2736 X 1824.

1. Launched chrome and made it maximized.
2. Added above 2 themes mentioned in comment#2 and didn't observe any theme issue.

@Reporter:Please check the video and let us know if we miss anything. Does this happen on every profile after adding theme? Any further help in reproducing the issue would be helpful.

cc'ing ramyan@ and kristipark@ from  issue 819781  for further inputs on this issue

Thanks!

Deleted: 822492.mp4 7.5 MB

	822492.mp4 7.5 MB View Download

sindhu.chelamcherla@ your Windows uses 200% scaling which means the effective resolution is half of the physical one: 2736/2 X 1824/2. To reproduce the bug you need to use 100% scaling in OS. This is the default setting for people with 27" (and bigger) displays. 

Video in comment 4 is spot on with the exception of screen resolution.  My resolution is 3840 x 2160.  Not sure if that matters or not.  After multiple times uninstalling and reinstalling I am still having issues. one thing I did notice is that on the google logo above the search bar is missing too.  I can highlight it but its not visible. I have also tried before signing into my profile and get the same result. 

Deleted: screen2.png 8.1 MB

	screen2.png 8.1 MB View Download

Thank you for providing more feedback. Adding the requester to the cc list.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Thanks for your reports. From what I can tell, these themes have the same behavior as in 64.0.3282.0. sindhu.chelamcherla@ can you confirm the same with 63?  

As part of  issue 812065 , we reverted to a prior layout, but I don't see background-size: cover in previous revisions. The themes mentioned in comment #2 specify their own element.style, which background-size won't override, afaict.

Commenter #6: what theme are you using?

>The themes mentioned in comment #2 specify their own element.style, which background-size won't override, afaict.

Themes can't specify their style (assuming you mean CSS) and never could.
It's the NTP's responsibility.

Regardless of recent changes in ssue 812065, I see two possible technical solutions:

1. stretch all themes' background by adding "body {background-size:cover}" to the NTP's CSS
   which might be undesirable for some themes, but generally acceptable

2. add a new NTP_BACKGROUND_ALIGNMENT in theme properties e.g. "ALIGN_COVER":
   https://cs.chromium.org/chromium/src/chrome/browser/themes/theme_properties.h?l=59
   which will obviously require the theme authors to update their themes

RE comment 8:

One I use is : https://goo.gl/P7nAez

Have also tried and got the same result:
https://goo.gl/TWMJAo
https://goo.gl/FYN9PU
https://goo.gl/K2fnpW

Re comment#8 tested on 63.0.3239.132 and observed same behavior as mentioned below.

Tested the issue on Windows 10 HIDPI 13 inch machine with screen resolution 3840 x 2160 with different scale factors. Able to reproduce this issue with above mentioned settings from 100% to 175% scaling. Issue is not reproducible from 200%. Attaching screenshots of behavior. 

NOTE: This can be seen on altering zoom levels as well. Issue is reproducible on Linux and Mac on zooming out NTP after adding any theme.

This issue is seen from M-60. Hence considering this issue as Non-Regression, marking as Untriaged and removing Needs-Biect label.

Deleted: 822492 with 3840x2160 & 175%.PNG 4.8 MB

	822492 with 3840x2160 & 175%.PNG 4.8 MB View Download

Deleted: 822492 with 3840x2160 & 200%.PNG 2.1 MB

	822492 with 3840x2160 & 200%.PNG 2.1 MB View Download

Deleted: 822492 with 150%.PNG 7.8 MB

	822492 with 150%.PNG 7.8 MB View Download

Deleted: 822492 with 125%.PNG 6.1 MB

	822492 with 125%.PNG 6.1 MB View Download

Deleted: 822492 with 100%.PNG 4.2 MB

	822492 with 100%.PNG 4.2 MB View Download

Thanks sindhu.chelamcherla!

Based on your comments, I've added a few more labels indicating the releases this is present in.

Assigning to kristipark@ to look at, as time permits.

It looks like the original issue was crbug/819781. Since that's fixed now, this is a duplicate of crbug/437245 and is working as intended.

Fixed links:
http://crbug/819781
http://crbug/437245

So a white page with a partial image in the middle with NO search bar is working as intended?  This is not a issue of just the bottom of the screen being white, therefore is not a duplicate of crbug/819781 and is definitely not fixed.  Chrome is fully up to date and problem persists.