By default, Bootstrap uses a solid color and dim the element by setting `$hr-opacity` to `0.25`[1]. Our design comes with a color that is "dimmed by default" since: ```scss $hr-color ← $border-color ← rgba(currentColor, .25) ``` This commit therefore sets `$hr-opacity: 1 !default`. At the same time, this also removes the hr background-color that Bootstrap sets, otherwise it conflicts with the transparent color we are adding on top of it. [1]: https://github.com/odoo/odoo/blob/1154556/addons/web/static/lib/bootstrap/scss/_variables.scss#L664 closes odoo/odoo#160495 Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com> Co-authored-by: qsm-odoo <qsm@odoo.com>