Rework card to be more neutral base theme, add GitHub theme. Fix list selection spacing

src/themes/packages/default/elements/list.variables

@@ -100,7 +100,7 @@
 @invertedLinkListItemActiveColor: @invertedSelectedTextColor;
 
 /* Selection List */
-@selectionListItemMargin: 0em 0em 0.3em 0em;
+@selectionListItemMargin: 0em;
 @selectionListItemBorderRadius: 0.5em;
 @selectionListItemVerticalPadding: 0.5em;
 @selectionListItemHorizontalPadding: 0.5em;

src/themes/packages/default/views/card.variables

@@ -7,7 +7,7 @@
 --------------------*/
 
 /* Shadow */
-@shadowDistance: 0em;
+@shadowDistance: 0.2em;
 @shadowBoxShadow: 0px @shadowDistance 0px 0px @solidBorderColor;
 
 /* Item */
@@ -16,7 +16,7 @@
 @display: block;
 @margin: 1em 0em;
 @minHeight: 0px;
-@padding: 0em;
+@padding: 0.5em;
 @width: 290px;
 @borderWidth: 1px;
 @borderShadow: 0px 0px 0px @borderWidth @solidBorderColor;
@@ -103,13 +103,13 @@
 
 /* Additional Content */
 @extraDivider: 1px solid rgba(0, 0, 0, 0.05);
-@extraBackground: #FAFAFA @subtleGradient;
+@extraBackground: none;
 @extraPosition: static;
 @extraTop: 0em;
 @extraLeft: 0em;
 @extraWidth: 100%;
 @extraPadding: 0.75em 1em;
-@extraBoxShadow: 0 1px 1px rgba(0, 0, 0, 0.15);
+@extraBoxShadow: none;
 @extraColor: @lightTextColor;
 @extraTransition: color @transitionDuration @transitionEasing;
 

src/themes/packages/github/views/card.overrides

@@ -0,0 +1,98 @@
+/*******************************
+             Item
+*******************************/
+/*-------------------
+         View
+--------------------*/
+
+/* Item */
+@background: #FFFFFF;
+@borderRadius: 0.325rem;
+@display: block;
+@float: left;
+@margin: 0em @horizontalSpacing @rowSpacing;
+@minHeight: 0px;
+@padding: 0em;
+@width: 300px;
+@boxShadow:
+  0px 0px 0px 1px @borderColor,
+  0px 3px 0px 0px @borderColor
+;
+@border: none;
+@zIndex: '';
+
+/* Item Group */
+@horizontalSpacing: 0.5em;
+@rowSpacing: 2.5em;
+@groupMargin: 1em -@horizontalSpacing;
+
+/*-------------------
+       Content
+--------------------*/
+
+/* Image */
+@imageBackground: @transparentBlack;
+@imagePadding: 0em;
+@imageBorderRadius: @borderRadius @borderRadius 0em 0em;
+@imageBoxShadow: none;
+@imageBorder: none;
+
+/* Content */
+@contentMargin: 0em;
+@contentPadding: 0.75em 1em;
+@contentFontSize: 1em;
+@contentBorder: none;
+@contentBorderRadius: 0em;
+@contentBoxShadow: none;
+
+/* Title */
+@titleMargin: 0em;
+@titleFont: @headerFont;
+@titleFontWeight: bold;
+@titleFontSize: 1.25em;
+@titleColor: @darkTextColor;
+
+/* Metadata */
+@metaColor: @lightTextColor;
+
+/* Description */
+@descriptionDistance: 0.75em;
+@descriptionColor: @lightTextColor;
+
+/* Image */
+@imageSpacing: 0.25em;
+@contentImageWidth: 2em;
+@contentImageVerticalAlign: middle;
+
+/* Paragraph */
+@paragraphDistance: 0.1em;
+
+/* Additional Content */
+@extraDisplay: absolute;
+@extraTop: 100%;
+@extraLeft: 0em;
+@extraWidth: 100%;
+
+@extraPadding: 0.5em 0.75em;
+@extraColor: @lightTextColor;
+@extraTransition: color @transitionDuration @transitionEasing;
+
+/*-------------------
+       States
+--------------------*/
+
+@hoverCursor: pointer;
+@hoverZIndex: 5;
+@hoverBorder: none;
+@hoverBoxShadow:
+  0px 0px 0px 1px @selectedBorderColor,
+  0px 3px 0px 0px @selectedBorderColor
+;
+
+
+/*-------------------
+      Variations
+--------------------*/
+
+/* Sizes */
+@medium: 1em;

src/themes/packages/github/views/card.variables

@@ -0,0 +1,22 @@
+/*******************************
+             Card
+*******************************/
+
+/*-------------------
+         View
+--------------------*/
+
+/* Shadow */
+@shadowDistance: 0em;
+@padding: 0em;
+
+/*-------------------
+       Content
+--------------------*/
+
+/* Additional Content */
+@extraDivider: 1px solid rgba(0, 0, 0, 0.05);
+@extraBackground: #FAFAFA @subtleGradient;
+@extraPadding: 0.75em 1em;
+@extraBoxShadow: 0 1px 1px rgba(0, 0, 0, 0.15);
+@extraColor: @lightTextColor;