Var: Allow transitions to be overridden

This allows to override transitions parameters (for example if you want them slower or faster) the same way you would tweak the primary/accent colors.

packages/theme-chalk/src/common/var.scss

@@ -2,12 +2,12 @@
 
 /* Transition
 -------------------------- */
-$--all-transition: all .3s cubic-bezier(.645,.045,.355,1);
-$--fade-transition: opacity 300ms cubic-bezier(0.23, 1, 0.32, 1);
-$--fade-linear-transition: opacity 200ms linear;
-$--md-fade-transition: transform 300ms cubic-bezier(0.23, 1, 0.32, 1) 100ms, opacity 300ms cubic-bezier(0.23, 1, 0.32, 1) 100ms;
-$--border-transition-base: border-color .2s cubic-bezier(.645,.045,.355,1);
-$--color-transition-base: color .2s cubic-bezier(.645,.045,.355,1);
+$--all-transition: all .3s cubic-bezier(.645,.045,.355,1) !default;
+$--fade-transition: opacity 300ms cubic-bezier(0.23, 1, 0.32, 1) !default;
+$--fade-linear-transition: opacity 200ms linear !default;
+$--md-fade-transition: transform 300ms cubic-bezier(0.23, 1, 0.32, 1) 100ms, opacity 300ms cubic-bezier(0.23, 1, 0.32, 1) 100ms !default;
+$--border-transition-base: border-color .2s cubic-bezier(.645,.045,.355,1) !default;
+$--color-transition-base: color .2s cubic-bezier(.645,.045,.355,1) !default;
 
 /* Colors
 -------------------------- */