Best combined with:
FT DeepDark add-on by steva (Stefano Roselli) (Firefox Only)

Dark Fusion - Google by DeathTBO - http://freestyler.ws/style/118354/dark-fusion-google

Remove "Search Google Keep" Text - User script I created to remove the search box placeholder text. https://greasyfork.org/en/scripts/12941-remove-search-google-keep-text

Google Keep Notes Firefox add-on -

DeepDark Theme by actualmanx (Chrome only) https://chrome.google.com/webstore/detail/deep-dark/eeffcpnmcmfdfnaadpnkldhkcjjiihcf

DeepDark Scroll Bars extension by actualmanx (Chrome only) https://chrome.google.com/webstore/detail/deep-dark-scrollbars/feeaojjpfjaggengmoigjlckepbmkane

03/10/2017 - Fixed slideout menu. Sorry for the delay!
01/20/2017 - Remove yellow color from notes. Google removed it so I'm getting rid of it too. Not worth keeping up with changes to the page to keep it. Fixes to sidebar to remove borders and darken text.
09/17/2016 - Fixed mobile slideout menu top section. Shift up notes container.
07/28/2016 - Fixed mobile slideout menu top section.
07/09/2016 - Some adjustments made for when keep web page is resized to a small width. Appears to be a dynamic resize for mobile web pages. Now looks correct with the Google Keep Notes firefox add-on. Updated theme screenshot.
06/30/2016 - Fixes for slight redesign by google. For now Google bar dark style is no longer required due to bar being merged into keep web design.
11/18/2015 - Lightened the shadow when hovering over a note item.
10/01/2015 - Fixed for updates to page coloring. Black button style now removed. Uncomment in style if you want to keep it. Fixed search dropdown color and styled dropdown arrow button.
06/19/2015 - Updated description. Older chrome theme now obsolete.
11/24/2014 - Fixed "Done" button styling for all note colors. Darkened link text in slideout menu and removed white divider line in slideout menu.
11/21/2014 - Colored some buttons, made note input yellow and slideout menu yellow also. Colored light bulb and text for empty views.
11/20/2014 - Initial release, mostly based off of my Google Voice style (http://freestyler.ws/style/55422/google-voice-dark-style)

This was tested in latest version of Chrome and Firefox.