Issue #3490336 by mably: CSS handling improvements

Closes #3490336

css/reading_progress_bar.css

 progress.reading-progress-bar {
   position: fixed;
+  z-index: 99;
   top: 0;
   left: 0;
   display: block;
   width: 100%;
+  height: var(--reading-progress-bar--height, 5px);
+  color: var(--reading-progress-bar--color, #0060df);
+  border: var(--reading-progress-bar--border, 0);
+  background-color: var(--reading-progress-bar--background-color, #e9e9ed);
   -webkit-appearance: none;
   -moz-appearance: none;
   appearance: none;
 }
 
+progress.reading-progress-bar::-webkit-progress-value {
+  background-color: var(--reading-progress-bar--color, #0060df);
+}
+progress.reading-progress-bar::-moz-progress-bar {
+  background-color: var(--reading-progress-bar--color, #0060df);
+}
+progress.reading-progress-bar::-webkit-progress-bar {
+  background-color: var(--reading-progress-bar--background-color, #e9e9ed);
+}
+
+.toolbar-vertical progress.reading-progress-bar,
+.toolbar-horizontal progress.reading-progress-bar {
+  top: 40px;
+}
+.toolbar-horizontal.toolbar-tray-open progress.reading-progress-bar {
+  top: 79px;
+}
+
 progress.reading-progress-bar {
   transition-duration: 0s, 1s;
   transition-property: visibility, opacity;