design-themes/theme_test_custo/data/shapes.xml

<?xml version="1.0" encoding="utf-8"?>
<odoo>

<!-- Background Shape -->
<template id="snippet_options_background_options" inherit_id="website.snippet_options_background_options" name="Background Shape Custom Option">
    <xpath expr="//we-select-pager[hasclass('o_we_shape_menu')]/we-select-page[last()]" position="after">
        <we-select-page string="Custom Shapes">
            <we-button data-shape="theme_test_custo/hexagons/01" data-select-label="Hexagon 01"/>
            <we-button data-shape="theme_test_custo/curves/01" data-select-label="Curve 01"/>
        </we-select-page>
    </xpath>
</template>
<record id="shape_hexagon_01" model="theme.ir.attachment">
    <field name="name">Hexagon Shape 01</field>
    <field name="key">theme_test_custo.shape_hexagon_01</field>
    <field name="url">/web_editor/shape/illustration/hexagons/01.svg</field>
</record>
<record id="shape_curve_01" model="theme.ir.attachment">
    <field name="name">Curve Shape 01</field>
    <field name="key">theme_test_custo.shape_curve_01</field>
    <field name="url">/web_editor/shape/illustration/curves/01.svg</field>
</record>

<!-- Image Shape -->
<template id="snippet_options" inherit_id="web_editor.snippet_options" name="Image Shape Custom Option">
    <xpath expr="//we-select-pager[@data-name='shape_img_opt']/we-select-page[last()]" position="after">
        <we-select-page string="Custom Shapes">
            <we-button data-set-img-shape="theme_test_custo/blob/01" data-select-label="Blob 01"/>
        </we-select-page>
    </xpath>
</template>
<record id="img_shape_blob_01" model="theme.ir.attachment">
    <field name="name">Blob Shape 01</field>
    <field name="key">theme_test_custo.img_shape_blob_01</field>
    <field name="url">/web_editor/image_shape/blob/01.svg</field>
</record>

</odoo>