refactor: move inline styles to table_widget.css and use cla… (#2357)

This update refactors how the interactive DataFrame widget (anywidget
mode) handles styling. It moves styling logic from Python code into
dedicated CSS classes, making the display system cleaner and more
robust.


Fixes #<issue_number_goes_here> 🦕

Author: shuoweil

bigframes/display/table_widget.css

@@ -15,133 +15,138 @@
  */
 
 .bigframes-widget {
-	display: flex;
-	flex-direction: column;
+  display: flex;
+  flex-direction: column;
 }
 
 .bigframes-widget .table-container {
-	max-height: 620px;
-	overflow: auto;
+  max-height: 620px;
+  overflow: auto;
 }
 
 .bigframes-widget .footer {
-	align-items: center;
-	/* TODO(b/460861328): We will support dark mode in a media selector once we
-	 * determine how to override the background colors as well. */
-	color: black;
-	display: flex;
-	font-family:
-		"-apple-system", "BlinkMacSystemFont", "Segoe UI", "Roboto", sans-serif;
-	font-size: 0.8rem;
-	justify-content: space-between;
-	padding: 8px;
+  align-items: center;
+  /* TODO(b/460861328): We will support dark mode in a media selector once we
+   * determine how to override the background colors as well. */
+  color: black;
+  display: flex;
+  font-family:
+    "-apple-system", "BlinkMacSystemFont", "Segoe UI", "Roboto", sans-serif;
+  font-size: 0.8rem;
+  justify-content: space-between;
+  padding: 8px;
 }
 
 .bigframes-widget .footer > * {
-	flex: 1;
+  flex: 1;
 }
 
 .bigframes-widget .pagination {
-	align-items: center;
-	display: flex;
-	flex-direction: row;
-	gap: 4px;
-	justify-content: center;
-	padding: 4px;
+  align-items: center;
+  display: flex;
+  flex-direction: row;
+  gap: 4px;
+  justify-content: center;
+  padding: 4px;
 }
 
 .bigframes-widget .page-indicator {
-	margin: 0 8px;
+  margin: 0 8px;
 }
 
 .bigframes-widget .row-count {
-	margin: 0 8px;
+  margin: 0 8px;
 }
 
 .bigframes-widget .page-size {
-	align-items: center;
-	display: flex;
-	flex-direction: row;
-	gap: 4px;
-	justify-content: end;
+  align-items: center;
+  display: flex;
+  flex-direction: row;
+  gap: 4px;
+  justify-content: end;
 }
 
 .bigframes-widget .page-size label {
-	margin-right: 8px;
+  margin-right: 8px;
 }
 
 .bigframes-widget table {
-	border-collapse: collapse;
-	/* TODO(b/460861328): We will support dark mode in a media selector once we
-	 * determine how to override the background colors as well. */
-	color: black;
-	text-align: left;
+  border-collapse: collapse;
+  /* TODO(b/460861328): We will support dark mode in a media selector once we
+   * determine how to override the background colors as well. */
+  color: black;
+  text-align: left;
 }
 
 .bigframes-widget th {
-	background-color: var(--colab-primary-surface-color, var(--jp-layout-color0));
-	padding: 0;
-	position: sticky;
-	top: 0;
-	z-index: 1;
+  background-color: var(--colab-primary-surface-color, var(--jp-layout-color0));
+  padding: 0;
+  position: sticky;
+  text-align: left;
+  top: 0;
+  z-index: 1;
 }
 
 .bigframes-widget .bf-header-content {
-	box-sizing: border-box;
-	height: 100%;
-	overflow: auto;
-	padding: 0.5em;
-	resize: horizontal;
-	width: 100%;
+  box-sizing: border-box;
+  height: 100%;
+  overflow: auto;
+  padding: 0.5em;
+  resize: horizontal;
+  width: 100%;
 }
 
 .bigframes-widget th .sort-indicator {
-	padding-left: 4px;
-	visibility: hidden;
+  padding-left: 4px;
+  visibility: hidden;
 }
 
 .bigframes-widget th:hover .sort-indicator {
-	visibility: visible;
+  visibility: visible;
 }
 
 .bigframes-widget button {
-	cursor: pointer;
-	display: inline-block;
-	text-align: center;
-	text-decoration: none;
-	user-select: none;
-	vertical-align: middle;
+  cursor: pointer;
+  display: inline-block;
+  text-align: center;
+  text-decoration: none;
+  user-select: none;
+  vertical-align: middle;
 }
 
 .bigframes-widget button:disabled {
-	opacity: 0.65;
-	pointer-events: none;
+  opacity: 0.65;
+  pointer-events: none;
 }
 
 .bigframes-widget .bigframes-error-message {
-	background-color: #fbe;
-	border: 1px solid red;
-	border-radius: 4px;
-	font-family:
-		"-apple-system", "BlinkMacSystemFont", "Segoe UI", "Roboto", sans-serif;
-	font-size: 14px;
-	margin-bottom: 8px;
-	padding: 8px;
+  background-color: #fbe;
+  border: 1px solid red;
+  border-radius: 4px;
+  font-family:
+    "-apple-system", "BlinkMacSystemFont", "Segoe UI", "Roboto", sans-serif;
+  font-size: 14px;
+  margin-bottom: 8px;
+  padding: 8px;
 }
 
 .bigframes-widget .cell-align-right {
-	text-align: right;
+  text-align: right;
 }
 
 .bigframes-widget .cell-align-left {
-	text-align: left;
+  text-align: left;
+}
+
+.bigframes-widget .null-value {
+  color: gray;
 }
 
 .bigframes-widget td {
-	padding: 0.5em;
+  padding: 0.5em;
 }
 
 .bigframes-widget tr:hover td,
 .bigframes-widget td.row-hover {
-	background-color: var(--colab-hover-surface-color, var(--jp-layout-color2));
+  background-color: var(--colab-hover-surface-color, var(--jp-layout-color2));
 }