server/apps/theming/css/theming.scss

/** Calculate luma as it is also used in OCA\Theming\Util::calculateLuma */
@function luma($c) {
	$-local-red: red(rgba($c, 1.0));
	$-local-green: green(rgba($c, 1.0));
	$-local-blue: blue(rgba($c, 1.0));

	@return (0.2126 * $-local-red  + 0.7152 * $-local-green + 0.0722 * $-local-blue) / 255;
}

.nc-theming-main-background {
	background-color: $color-primary;
}

.nc-theming-main-text {
	color: $color-primary-text;
}

.nc-theming-contrast {
	color: $color-primary-text;
}

@if (luma($color-primary) > 0.6) {
	#appmenu:not(.inverted) svg {
		filter: invert(1);
	}
	#appmenu.inverted svg {
		filter: none;
	}
	.searchbox input[type="search"] {
		background: transparent url('../../../core/img/actions/search.svg') no-repeat 6px center;
	}
	#contactsmenu .icon-contacts {
		background-image: url('../../../core/img/places/contacts-dark.svg');
	}
	#settings .icon-settings-white {
		background-image: url('../../../core/img/actions/settings-dark.svg');
	}
	#appmenu .icon-more-white {
		background-image: url('../../../core/img/actions/more.svg');
	}

	#body-login {

		input,
		#alternative-logins li a {
			border: 1px solid nc-lighten($color-primary-text, 50%);
		}
		input.primary,
		#alternative-logins li a {
			background-color: $color-primary;
		}
		a,
		label,
		p,
		#alternative-logins legend {
			color: $color-primary-text !important;
		}
		input[type='checkbox'].checkbox--white + label:before {
			border-color: nc-darken($color-primary-element, 40%) !important;
		}
		input[type='checkbox'].checkbox--white:not(:disabled):not(:checked) + label:hover:before,
		input[type='checkbox'].checkbox--white:focus + label:before {
			border-color: nc-darken($color-primary-element, 30%) !important;
		}
		input[type='checkbox'].checkbox--white:checked + label:before {
			border-color: nc-darken($color-primary-element, 30%) !important;
			background-image: url('../../../core/img/actions/checkbox-mark.svg');
			background-color: nc-darken($color-primary-element, 30%) !important;
		}
	}
} @else {
	#appmenu:not(.inverted) svg {
		filter: none;
	}
	#appmenu.inverted svg {
		filter: invert(1);
	}
}

/* Colorized svg images */
.icon-file, .icon-filetype-text {
	background-image: url(./img/core/filetypes/text.svg?v=#{$theming-cachebuster});
}

.icon-folder, .icon-filetype-folder {
	background-image: url(./img/core/filetypes/folder.svg?v=#{$theming-cachebuster});
}

.icon-filetype-folder-drag-accept {
	background-image: url(./img/core/filetypes/folder-drag-accept.svg?v=#{$theming-cachebuster}) !important;
}

/* override styles for login screen in guest.css */
@if variable_exists('theming-logo-mime') and $theming-logo-mime != '' {
    #theming-preview-logo,
    #header .logo {
		background-image: url(#{$image-logo});
		background-size: contain;
	}
	#body-login #header .logo {
		margin-bottom: 22px;
	}
} @else {
  #theming-preview-logo {
    background-image: url(#{$image-logo});
  }
}

@if variable_exists('theming-background-mime') and $theming-background-mime != ''  {
	#body-login,
	#firstrunwizard .firstrunwizard-header,
	#theming-preview {
		background-image: url(#{$image-login-background});
		background-color: $color-primary;
	}
} @else {
	  #theming-preview {
		  background-image: url(#{$image-login-background});
		  background-color: $color-primary;
	  }
}

@if variable_exists('theming-logoheader-mime') and $theming-logoheader-mime != '' {
	#theming .advanced-option-logoheader .image-preview,
	body:not(#body-login) #header .logo {
		background-image: url(#{$image-logoheader});
	}
} @else {
	#theming .advanced-option-favicon .image-preview {
		background-image: none;
	}
}

@if variable_exists('theming-favicon-mime') and $theming-favicon-mime != '' {
	#theming .advanced-option-favicon .image-preview {
		background-image: url(#{$image-favicon});
	}
} @else {
	#theming .advanced-option-favicon .image-preview {
		background-image: none;
	}
}

input.primary,
#alternative-logins li a {
	background-color: $color-primary-element;
	border: 1px solid $color-primary-text;
	color: $color-primary-text;
}

@if (luma($color-primary) > 0.6) {
	#body-login #submit-wrapper .icon-confirm-white {
		background-image: url('../../../core/img/actions/confirm.svg');
	}
}

// plain background color for login page
@if $image-login-plain == 'true' {
	#body-login, #firstrunwizard .firstrunwizard-header, #theming-preview {
		background-image: none !important;
		background-color: $color-primary;
	}
	#body-login {

		a, label, p {
			color: $color-primary-text !important;
		}

	}
}

@if ($color-primary == #ffffff) {
	/* show grey border below header */
	#body-user #header,
	#body-settings #header,
	#body-public #header {
		border-bottom: 1px solid #ebebeb;
	}

	/* show triangle in header in grey */
	#appmenu li a.active:before,
	.header-right #settings #expand:before {
		border-bottom-color:#ebebeb;
	}

	/* show border around quota bar in files app */
	.app-files #quota .quota-container {
		border: 1px solid #ebebeb;
	}
}