like this:
type: custom:mushroom-title-card
title: Hello, {{ user }} !
subtitle: Test
card_mod:
style: |
ha-card {
--title-font-weight: 900;
--subtitle-font-weight: 900;
}
or like this:
card_mod:
style: |
.title {
font-weight: 900 !important;
}
.subtitle {
font-weight: 900 !important;
}
i always prefer to edit it directly through the element than the variable though.
font weights tend to max out at 900 btw:
Should probably be in my guide tbh. ill see if i can add it. because its not mentioned in the title card section:
(i know it says template card in the preview, but trust me title card is in the same post 
)