Windows:
C:\Program Files\Inductive Automation\Ignition\data\modules\com.inductiveautomation.perspective\themes
macOS (similar for Linux):
/usr/local/ignition/data/modules/com.inductiveautomation.perspective/themes
Without any changes to that file, you want line 146:
.button-secondary {
font-family: 'Roboto', sans-serif;
font-weight: 400;
font-size: 0.875rem;
color: #000000;
-webkit-border-radius: 6px;
border-radius: 6px;
background-clip: padding-box;
border: 1px solid #C1C9D0;
background-color: #D7DBDE;
background-size: 101% 101%;
color: #70757A;
font-weight: 700;
display: inline-block;
text-decoration: none; }
.button-secondary img {
vertical-align: middle; }
.button-secondary.icon-left img {
padding-right: 0.25rem; }
.button-secondary.icon-right img {
padding-left: 0.25rem; }
.button-secondary svg {
fill: #70757A; }
.button-secondary:hover, .button-secondary:active {
background-color: #D6DBDE;
cursor: pointer; }
.button-secondary:hover {
box-shadow: 0 2px 4px 0 rgba(34,34,34,0.4); }
.button-secondary:active {
box-shadow: inset 0 0 8px 4px rgba(34,34,34,0.4); }
.button-secondary:disabled {
pointer-events: none;
background-color: #DDE1E4; }