fix: enhance layout consistency in PluginInstall.page and default-base.css

- Added utility classes for width, height, and overflow to improve layout structure in PluginInstall.page.
- Removed the obsolete Plugins.css file as its styles are no longer needed.
- This change continues the effort to enhance visual consistency across the plugin.

emhttp/plugins/dynamix/styles/default-base.css

@@ -1870,6 +1870,35 @@ span#wlan0 {
 .justify-self-stretch {
     justify-self: stretch;
 }
+
+.w-full {
+    width: 100%;
+}
+/* matches Tailwind scale */
+.max-w-20 {
+    max-width: 80rem; /* 1rem = 4. 80/4 = 20. 1rem = 10px with current font-size */
+}
+
+.mx-auto {
+    margin-left: auto;
+    margin-right: auto;
+}
+
+.h-50 {
+    height: 20rem;
+}
+
+.overflow-y-auto {
+    overflow-y: auto;
+}
+
+.overflow-y-scroll {
+    overflow-y: scroll;
+}
+
+.scrollbar-thin {
+    scrollbar-width: thin;
+}
 /*
  * Using CSS Nesting, to narrow down the scope of the styles to the .Theme--sidebar class.
  * This allows us to have default-azure & default-gray set css variables