Posting this over here, so anyone can find it if they use this theme:
Took a quick look at Google web fonts. Without going through every possibility, it appears the bold variant of the Arsenal font is an extremely close match to the font Tamuril used in the original images. I doubt most people would be able to pick the difference unless they were really trying to.
So, IMO the best option would be to call in Arsenal from Google and use that instead of the original menu images. With suitable choice of colour and text-shadow it should be almost indistinguishable from the original, and would make adding buttons a piece of cake. I'll play around with it later when I have more time.
Have played around with it, and it's a simple substitution. Turns out the
Fira Sans Extra Condensed font is a better match than Arsenal. Arsenal doesn't look quite right in either normal or bold font weight, and some browsers won't support the semi-bold variant. Fira Sans Extra Condensed looks almost identical to Tamuril's original buttons if set to normal font weight. So, code:
#navigation {
width: 100%;
min-width: 820px;
background: #e9e6e1 url(../images/theme/gf-menubar.png) repeat-x;
height: 39px;
padding: 0;
margin: 0;
display: block;
white-space:nowrap;
}
.navlevel{
margin-right: 8px;
}
#back_admin {
background: #e9e6e1 url(../images/theme/gf-admin.png) no-repeat;
width: 77px;
height: 39px;
}
#back_admin span{
display: none;
}
#back_home {
background: #e9e6e1 url(../images/theme/gf-home.png) no-repeat;
width: 92px;
height: 39px;
}
#back_home span{
display: none;
}
#back_help {
background: #e9e6e1 url(../images/theme/gf-help.png) no-repeat;
width: 59px;
height: 39px;
}
#back_help span{
display: none;
}
#back_search {
background: #e9e6e1 url(../images/theme/gf-search.png) no-repeat;
width: 76px;
height: 39px;
}
#back_search span{
display: none;
}
#back_moderate {
background: #e9e6e1 url(../images/theme/gf-moderate.png) no-repeat;
width: 107px;
height: 39px;
}
#back_moderate span{
display: none;
}
#back_profile {
background: #e9e6e1 url(../images/theme/gf-profile.png) no-repeat;
width: 79px;
height: 39px;
}
#back_profile span{
display: none;
}
#back_mlist {
background: #e9e6e1 url(../images/theme/gf-members.png) no-repeat;
width: 100px;
height: 39px;
}
#back_mlist span{
display: none;
}
#back_pm {
background: #e9e6e1 url(../images/theme/gf-messages.png) no-repeat;
width: 133px;
height: 39px;
}
#back_pm span{
display: none;
}
#back_logout {
background: #e9e6e1 url(../images/theme/gf-logout.png) no-repeat;
width: 79px;
height: 39px;
}
#back_logout span{
display: none;
}
#back_login {
background: #e9e6e1 url(../images/theme/gf-login.png) no-repeat;
width: 66px;
height: 39px;
}
#back_login span{
display: none;
}
#back_register {
background: #e9e6e1 url(../images/theme/gf-register.png) no-repeat;
width: 86px;
height: 39px;
}
#back_register span{
display: none;
}
#navigation {
min-width: 820px;
padding-left: 1em;
background: #e9e6e1 url(../images/theme/gf-menubar.png) repeat-x;
height: 39px;
white-space:nowrap;
}
.dropmenu a span {
display: block;
padding: 0 3px;
font-size: 0.9em;
}
.dropmenu a>.navlevel {
padding: 0 5px;
line-height: 39px;
color: #fefffb;
font-family: "Fira Sans Extra Condensed", sans-serif;
font-size: 18px;
text-transform: lowercase;
letter-spacing: 1px;
text-shadow: 0 0 4px #566341;
}
/* Fixes for minor RC3 bugs. */
#main_admsection {
position: relative;
left: 0;
right: 0;
overflow: hidden;
}
#adm_submenus, #adm_submenus ul {
padding-left: 1em;
}
Which saves a lot of stuff.

(The last two declarations are for fixing the small RC3 bugs that were in this theme, due to the state of 2.0.x dev builds at the time.)
You'll also need a small edit to index.template.php:
<link rel="stylesheet" type="text/css" href="', $settings['theme_url'], '/css/gfstyle', $context['theme_variant'], '.css?rc3" />
<link rel="stylesheet" type="text/css" href="https://fonts.googleapis.com/css?family=Fira+Sans+Extra+Condensed&display=swap">
<link rel="stylesheet" type="text/css" href="', $settings['theme_url'], '/css/gfstyle', $context['theme_variant'], '.css?rc3" />
The result will look almost identical to the original, but will allow adding custom buttons without problems. Screenshot attached. Obviously it can be re-styled any way you like if you want a different look.