preference: Fix visually broken delete custom css button.

There were couple of issue solved by this commit:
  - The button had extra 10px margin to right; this comes from .action class
  - The "Delete" text and icon were not aligned
  - The button looks bigger than other buttons

Both of which made the button look broken. We fix first by changing
.css-delete-action to #css-delete-action in css file; git grep shows
this is only used once and is used as id not a class. Then we use
display flex to fix the alinging of icon and text. We add box-sizing border-box
which solves the bigger width issue. We remove extra 10px margin that comes from
.action.

Fixes: #801.

app/renderer/css/preference.css

@@ -399,8 +399,17 @@ i.open-tab-button {
     cursor: pointer;
 }
 
-.css-delete-action {
-    margin-bottom: 10px;
+#css-delete-action {
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    box-sizing: border-box;
+
+    /*
+        .action class will add extra margin to right which
+        we don't want for a button; the extra margin is intended for radio buttons
+    */
+    margin-right: 0px;
 }
 
 .selected-css-path,

app/renderer/js/pages/preference/general-section.ts

@@ -104,7 +104,6 @@ class GeneralSection extends BaseSection {
 				<button class="custom-css-button green">${t.__('Upload')}</button>
 			</div>
 			<div class="setting-row" id="remove-custom-css">
-		"S
 				<div class="setting-description">
 					<div class="selected-css-path" id="custom-css-path">${ConfigUtil.getConfigItem('customCSS')}</div>
 				</div>