Added Ubuntu theme to the internal packages

internal_packages/ui-ubuntu/README.md

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+# Ubuntu Theme for Nylas N1 #
+
+![img](https://raw.githubusercontent.com/ahmedlhanafy/Ubuntu-Ui-Theme-for-Nylas-N1/master/Screenshot.png)
+
+## Installation: ##
+
+* Download the zip folder and extract it.
+* Update N1 to the latest version go to Preferences -> General -> Select theme -> Install a theme and then select the extracted folder.

internal_packages/ui-ubuntu/package.json

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+{
+  "name": "ui-ubuntu",
+  "displayName": "Ubuntu",
+  "theme": "ui",
+  "version": "0.1.0",
+  "description": "Ubuntu Theme for Nylas N1",
+  "license": "Proprietary",
+  "engines": {
+    "atom": "*"
+  },
+  "private": true
+}

internal_packages/ui-ubuntu/styles/theme-colors.less

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+@component-active-color: #f07746;
+@toolbar-background-color: #41403b;

internal_packages/ui-ubuntu/styles/ui-variables.less

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+
+//buttons color
+@btn-default-bg-color:      lighten(@toolbar-background-color,4%);
+
+// dropdowns color
+@dropdown-default-bg-color:#f07746;
+
+@accent-primary:            #f07746;
+@accent-primary-dark:       darken(#f07746, 1%);
+
+@border-secondary-bg:   lighten(@background-secondary, 10%);
+
+
+@toolbar-background-color: #41403b;
+@light:  rgb(246, 246, 246);
+
+.sheet-toolbar .btn-toolbar img.content-mask {
+    background-color: @light;
+}
+
+.layout-mode-popout .sheet-toolbar {
+display: none;
+}
+
+.sheet-toolbar-container {
+  background-image: -webkit-linear-gradient(top,@toolbar-background-color, darken(@toolbar-background-color,5%));
+    box-shadow: none;
+}
+
+.sheet-toolbar .item-back img.content-mask{
+  background-color: @light;
+}
+
+.sheet-toolbar .item-back .item-back-title{
+  color: @light;
+}
+
+.btn-icon img.content-mask {
+    background-color:@light;
+}
+
+.btn-icon img.content-mask:hover {
+    background-color:@accent-primary;
+}