[FIX] theme_*: fix s_big_number wrong implementation

*: anelusia, artists, avantgarde, aviato, beauty, bewise, bistro,
bookstore, buzzy, clean, cobalt, enark, graphene, kea, kiddo, loftspace,
monglia, nano, notes, odoo_experts, orchid, paptic, real_estate,
treehouse, vehicle, yes, zap

This commit aims to fix a wrong implementation of gradients on the
`s_big_number` snippet within the context of the web editor.

Prior to this commit, the `s_big_number` snippet was using a gradient
composed of the right color, but being declared as `RGB` values.

While this is working as expected and looking like it uses the right
`--o-color-*`, it's indeed working with the specific color applied in
first place. If a user changes the color palette of the website, the
snippet will look terrible as it doesn't adapt to the new palette.

This commit fixes the issue by using the right implementation with CSS
variables.

It also reviews a wrong structure using `<div>` while it should use the
right html tag.

task-4202371

closes odoo/design-themes#930

Related: odoo/odoo#180983
Signed-off-by: Benjamin Vray (bvr) <bvr@odoo.com>

theme_kea/views/snippets/s_big_number.xml

@@ -14,10 +14,10 @@
     </xpath>
 
     <!-- Text -->
-    <xpath expr="//h2/div" position="replace" mode="inner">
+    <xpath expr="//h2/span" position="replace" mode="inner">
         80+
     </xpath>
-    <xpath expr="//h2/following-sibling::div" position="replace" mode="inner">
+    <xpath expr="//p/span" position="replace" mode="inner">
         satisfied clients
     </xpath>
 </template>