[FINNA-1432] Define base font size for the html element. (#3100)

Allows use of relative font sizes and paves way for Bootstrap 5 migration.

themes/finna2/scss/finna-other.scss

@@ -65,6 +65,10 @@
 @import 'finna/select-a11y';
 @import 'finna/reservationlist';
 
+html {
+    font-size: $font-size-base;
+}
+
 ::-ms-clear {
     display: none;
 }