[Plugin: BM Custom Login] change the color of some part

edit bm-custom-login-3.css

Add:

input.button-primary {
background:#21759B;
border-color:#298CBA;
color:#FFFFFF;
font-weight:bold;
text-shadow:0 -1px 0 rgba(0, 0, 0, 0.3);
}
input.button-primary:active {
background:#21759B;
color:#EAF2FA;
}
input.button-primary:hover {
border-color:#13455b;
color:#eaf2fa;
}

.login .message {
background-color:#FFFFE0;
border-color:#E6DB55;
}

Change the color #hex values to whatever you like.

Please note that this will be overwritten if/when there is an update to the plugin (usually with major WordPress version updates).

font-weight:bold;
text-shadow:0 -1px 0 rgba(0, 0, 0, 0.3);

those lines are not necessary

Example:
https://mainfarmersmarket.com/images/msfm-login-example.png