Settings -> Activate Developer Mode -> Technical -> Views, search View Architecture for "custom.web", there should be these two views:

/website/static/src/scss/options/colors/user_theme_color_palette.custom.web.assets_common.scss
/website/static/src/scss/options/user_values.custom.web.assets_common.scss

If they are not there, recreate them with values below will fix the issue, if they are there, then something else is wrong:

----------------------------------------------------
View Name
/website/static/src/scss/options/colors/user_theme_color_palette.custom.web.assets_common.scss

View Type
QWeb

Key
web_editor.scss_54e7fe

Sequence
16

Inherited View
_assets_primary_variables

View inheritance mode
Extension View

Architecture
<data inherit_id="website._assets_primary_variables" name="/website/static/src/scss/options/colors/user_theme_color_palette.custom.web.assets_common.scss">
<xpath expr="//link[@href='/website/static/src/scss/options/colors/user_theme_color_palette.scss']" position="attributes">
<attribute name="href">/website/static/src/scss/options/colors/user_theme_color_palette.custom.web.assets_common.scss</attribute>
</xpath>
</data>

--------------------------------------------------------------
View Name
/website/static/src/scss/options/user_values.custom.web.assets_common.scss

View Type
QWeb

Key
web_editor.scss_8e0d16

Sequence
16

Inherited View
_assets_primary_variables

View inheritance mode
Extension View

Architecture
<data inherit_id="website._assets_primary_variables" name="/website/static/src/scss/options/user_values.custom.web.assets_common.scss">
<xpath expr="//link[@href='/website/static/src/scss/options/user_values.scss']" position="attributes">
<attribute name="href">/website/static/src/scss/options/user_values.custom.web.assets_common.scss</attribute>
</xpath>
</data>