How do I add more columns in theme “mercury”?

I have not tested this, but I think you can do this:

(Do you want to do it nice or ugly?)
Ugly: set the front page layout to 4 columns, and then go to line 1525 in mercury>assets>css>main.css and set your width.
Nice: add your new option to the config.json and copy-past line 1513-1527 in the mentioned main.css and change the ‘–4’ ending to your new option value, and change the widths.

But your font size will not change, maybe also take a look at line 1608-1643.

This is in mercery v.2.2.0.0  and this is the lines in main.css:

@media all and (min-width: 37.5em) {
  .l-col--4 > .c-card {
    width: 50%;
  }
}
@media all and (min-width: 56.25em) {
  .l-col--4 > .c-card {
    width: 33.333%;
  }
}
@media all and (min-width: 75em) {
  .l-col--4 > .c-card {
    width: 25%;
  }
}

...


.c-card__text > p {
  font-size: 0.624295077rem;
  letter-spacing: 0.09em;
  margin-top: 0.8rem;
  text-transform: uppercase;
}
.c-card__text > h2 {
  font-size: 1.125rem;
  line-height: 1.4;
  margin-top: 0.5333333333rem;
}

.c-card__text--compact > p {
  font-size: 0.624295077rem;
}
.c-card__text--compact > h2 {
  font-size: 0.8888888889rem;
  font-weight: 700;
  line-height: 1.3;
  margin-top: 0.5333333333rem;
}

hi, noob question how do you access the theme files?